Lowercase some Windows headers.

MinGW on Linux needs lowercase include files. On Windows this doesn't
matter since the filesystems are case-insensitive, but building
BoringSSL on Linux with MinGW has case-sensitive filesystems.

Change-Id: Id9c120d819071b041341fbb978352812d6d073bc
Reviewed-on: https://boringssl-review.googlesource.com/4090
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/mem.c b/crypto/mem.c
index a8a8c08..ce41440 100644
--- a/crypto/mem.c
+++ b/crypto/mem.c
@@ -66,7 +66,7 @@
 
 #if defined(OPENSSL_WINDOWS)
 #pragma warning(push, 3)
-#include <Windows.h>
+#include <windows.h>
 #pragma warning(pop)
 #else
 #include <strings.h>