Add remove_unused_interface_variable_pass.* to BUILD.gn (#4363)

diff --git a/BUILD.gn b/BUILD.gn
index 19ee77a..9587f6a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -676,6 +676,8 @@
     "source/opt/relax_float_ops_pass.h",
     "source/opt/remove_duplicates_pass.cpp",
     "source/opt/remove_duplicates_pass.h",
+    "source/opt/remove_unused_interface_variables_pass.cpp",
+    "source/opt/remove_unused_interface_variables_pass.h",
     "source/opt/replace_invalid_opc.cpp",
     "source/opt/replace_invalid_opc.h",
     "source/opt/scalar_analysis.cpp",