Fix building with clang targeting x86_32 mingw

In these cases, CLANG_HAS_AVX2 is defined, but _MSC_VER isn't.

The corresponding codepaths are in source/compare_win.cc, and the whole
file is guarded with a defined(_MSC_VER).

This is the same fix as in d8680893,
but this issue wasn't noticed since compare.o wasn't linked in in
earlier tested configurations.

Change-Id: Ifff6d4bd7b1071256df1169881490a52026c183b
Reviewed-on: https://chromium-review.googlesource.com/988573Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>

include/libyuv/compare_row.h

@@ -46,7 +46,7 @@ extern "C" {
 #endif  // __clang__
 
 // The following are available for Visual C:
-#if !defined(LIBYUV_DISABLE_X86) && defined(_M_IX86) && \
+#if !defined(LIBYUV_DISABLE_X86) && defined(_M_IX86) && defined(_MSC_VER) && \
     (defined(VISUALC_HAS_AVX2) || defined(CLANG_HAS_AVX2))
 #define HAS_HASHDJB2_AVX2
 #endif