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/row_win.cc, and the whole
file is guarded with an defined(_MSC_VER).

Change-Id: Idf0cfced3a0ef28c07b1393fe605a904420fd50f
Reviewed-on: https://chromium-review.googlesource.com/983592Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>

include/libyuv/row.h

@@ -241,7 +241,7 @@ extern "C" {
 
 // The following are available for AVX2 Visual C and clangcl 32 bit:
 // TODO(fbarchard): Port to gcc.
-#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_ARGB1555TOARGBROW_AVX2
 #define HAS_ARGB4444TOARGBROW_AVX2