Fix BUILD.gn for recent linker changes

PR #6211 added new source files but did not update BUILD.gn

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 3fa9d52..35b6034 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -657,7 +657,11 @@
 }
 
 static_library("spvtools_link") {
-  sources = [ "source/link/linker.cpp" ]
+  sources = [
+    "source/link/fnvar.cpp",
+    "source/link/fnvar.h",
+    "source/link/linker.cpp",
+  ]
   deps = [
     ":spvtools",
     ":spvtools_opt",