Add missing headers to GN. (#3009)

Detected via ANGLE's export_targets.py.

Fixes #3008
diff --git a/BUILD.gn b/BUILD.gn
index 2418e0b..5c8eb9a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -295,6 +295,14 @@
   }
 }
 
+source_set("spv_headers") {
+  sources = [
+    "$spirv_headers/include/spirv/1.2/GLSL.std.450.h",
+    "$spirv_headers/include/spirv/unified1/OpenCL.std.h",
+    "$spirv_headers/include/spirv/unified1/spirv.h",
+  ]
+}
+
 source_set("spvtools_headers") {
   sources = [
     "include/spirv-tools/instrument.hpp",
@@ -324,16 +332,22 @@
     "source/assembly_grammar.h",
     "source/binary.cpp",
     "source/binary.h",
+    "source/cfa.h",
     "source/diagnostic.cpp",
     "source/diagnostic.h",
     "source/disassemble.cpp",
+    "source/disassemble.h",
     "source/enum_set.h",
     "source/enum_string_mapping.cpp",
+    "source/enum_string_mapping.h",
     "source/ext_inst.cpp",
     "source/ext_inst.h",
     "source/extensions.cpp",
     "source/extensions.h",
     "source/instruction.h",
+    "source/latest_version_glsl_std_450_header.h",
+    "source/latest_version_opencl_std_header.h",
+    "source/latest_version_spirv_header.h",
     "source/libspirv.cpp",
     "source/macro.h",
     "source/name_mapper.cpp",
@@ -379,6 +393,7 @@
   ]
 
   public_deps = [
+    ":spv_headers",
     ":spvtools_core_enums_unified1",
     ":spvtools_headers",
   ]
@@ -393,8 +408,12 @@
 static_library("spvtools_val") {
   sources = [
     "source/val/basic_block.cpp",
+    "source/val/basic_block.h",
     "source/val/construct.cpp",
+    "source/val/construct.h",
+    "source/val/decoration.h",
     "source/val/function.cpp",
+    "source/val/function.h",
     "source/val/instruction.cpp",
     "source/val/validate.cpp",
     "source/val/validate.h",
@@ -425,14 +444,17 @@
     "source/val/validate_logicals.cpp",
     "source/val/validate_memory.cpp",
     "source/val/validate_memory_semantics.cpp",
+    "source/val/validate_memory_semantics.h",
     "source/val/validate_misc.cpp",
     "source/val/validate_mode_setting.cpp",
     "source/val/validate_non_uniform.cpp",
     "source/val/validate_primitives.cpp",
     "source/val/validate_scopes.cpp",
+    "source/val/validate_scopes.h",
     "source/val/validate_small_type_uses.cpp",
     "source/val/validate_type.cpp",
     "source/val/validation_state.cpp",
+    "source/val/validation_state.h",
   ]
 
   deps = [
@@ -720,6 +742,7 @@
     "source/reduce/reducer.h",
     "source/reduce/reduction_opportunity.cpp",
     "source/reduce/reduction_opportunity.h",
+    "source/reduce/reduction_opportunity_finder.h",
     "source/reduce/reduction_pass.cpp",
     "source/reduce/reduction_pass.h",
     "source/reduce/reduction_util.cpp",
@@ -872,7 +895,9 @@
     "tools/util/cli_consumer.cpp",
     "tools/util/cli_consumer.h",
   ]
-  deps = [ ":spvtools_headers" ]
+  deps = [
+    ":spvtools_headers",
+  ]
   configs += [ ":spvtools_internal_config" ]
 }