Surround immintrin.h includes with warning pragmas.

smmintrin.h uses #if __cplusplus rather than #ifdef __cplusplus which
trips up warnings.

Change-Id: Ic2e0a64f0485a6e1d807c1e2d9d7f5bf8c64e296
Reviewed-on: https://boringssl-review.googlesource.com/5450
Reviewed-by: Adam Langley <agl@google.com>

crypto/cpu-intel.c

diff --git a/crypto/cpu-intel.c b/crypto/cpu-intel.c
index a50d958..924bab0 100644
--- a/crypto/cpu-intel.c
+++ b/crypto/cpu-intel.c
@@ -69,8 +69,10 @@
 #include <string.h>
 
 #if defined(OPENSSL_WINDOWS)
+#pragma warning(push, 3)
 #include <immintrin.h>
 #include <intrin.h>
+#pragma warning(pop)
 #endif