vpx_ports/x86.h: fix visual studio build warning

test for WINAPI_FAMILY_PARTITION before using it

Change-Id: I6b7a7b1ccd3332b31c73b733f88c997e30b22de8

vpx_ports/x86.h

diff --git a/vpx_ports/x86.h b/vpx_ports/x86.h
index 0fef6a5..5da346e 100644
--- a/vpx_ports/x86.h
+++ b/vpx_ports/x86.h
@@ -136,7 +136,7 @@
 #define xgetbv() 0U  // no AVX for older x64 or unrecognized toolchains.
 #endif
 
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) && _MSC_VER >= 1700
 #include <windows.h>
 #if WINAPI_FAMILY_PARTITION(WINAPI_FAMILY_APP)
 #define getenv(x) NULL