Add custom extension for compiled javascript to copyright check exemption.

jenkins/check_copyright.sh

@@ -37,6 +37,7 @@ git diff --name-only `git merge-base origin/master HEAD` HEAD |
     grep -v -E '\.man$' |
     grep -v -E 'test/WasmSpec/testsuite/.*$' |
     grep -v -E 'test/benchmarks/.*\.js$' |
+    grep -v -E 'test/benchmarks/.*\.js_c$' |
     grep -v -E 'bin/External/.*$' |
     grep -v -E 'bin/NativeTests/Scripts/splay.js$' |
     grep -v -E 'pal/.*' |