Use the correct case for Windows headers.

When cross-compiling for Windows on a Linux system, the filesystem is
case sensitive and so #includes with uppercase characters cause errors.

Change-Id: I6fe6d677887ce84ba8b2c3136cf2c70998d96e81
Reviewed-on: https://boringssl-review.googlesource.com/5060
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

crypto/rand/windows.c

@@ -27,7 +27,7 @@
  * "Community Additions" comment on MSDN here:
  * http://msdn.microsoft.com/en-us/library/windows/desktop/aa387694.aspx */
 #define SystemFunction036 NTAPI SystemFunction036
-#include <NTSecAPI.h>
+#include <ntsecapi.h>
 #undef SystemFunction036
 
 #pragma warning(pop)

crypto/thread_test.c

@@ -22,7 +22,7 @@
 #if defined(OPENSSL_WINDOWS)
 
 #pragma warning(push, 3)
-#include <Windows.h>
+#include <windows.h>
 #pragma warning(pop)
 
 typedef HANDLE thread_t;