[gn build] copy compiler-rt headers to clang include dir on all platforms

llvm/utils/gn/secondary/BUILD.gn

@@ -7,6 +7,7 @@ group("default") {
     "//clang-tools-extra/clangd/test",
     "//clang-tools-extra/test",
     "//clang/test",
+    "//compiler-rt/include",
     "//compiler-rt/lib/scudo",
     "//lld/test",
     "//llvm/test",