Enable -Wbad-function-cast.

Fixes a small bug in xxhash.

Bug: skia:7647
Change-Id: Ibd0bd8dc4febfe585e257786d9683ea09c94ec6a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2376718
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

BUILD.gn

@@ -142,6 +142,7 @@ config("extra_warnings") {
   }
   if (is_clang) {
     cflags += [
+      "-Wbad-function-cast",
       "-Wconditional-uninitialized",
       "-Wextra-semi-stmt",
       "-Wfloat-conversion",

src/common/third_party/xxhash/xxhash.c

@@ -222,7 +222,7 @@ static int XXH_isLittleEndian(void)
     const union { U32 u; BYTE c[4]; } one = { 1 };   /* don't use static : performance detrimental  */
     return one.c[0];
 }
-#   define XXH_CPU_LITTLE_ENDIAN   XXH_isLittleEndian()
+#   define XXH_CPU_LITTLE_ENDIAN   (XXH_isLittleEndian())
 #endif