Fix includes on MinGW-w64 (#200)

diff --git a/rpmalloc/rpmalloc.c b/rpmalloc/rpmalloc.c
index e1cafc2..f3a30db 100644
--- a/rpmalloc/rpmalloc.c
+++ b/rpmalloc/rpmalloc.c
@@ -120,9 +120,9 @@
 #  ifndef WIN32_LEAN_AND_MEAN
 #    define WIN32_LEAN_AND_MEAN
 #  endif
-#  include <Windows.h>
+#  include <windows.h>
 #  if ENABLE_VALIDATE_ARGS
-#    include <Intsafe.h>
+#    include <intsafe.h>
 #  endif
 #else
 #  include <unistd.h>