win32: define DWORD_MAX if it's not defined

MinGW does not define DWORD_MAX.  Specify it when it's not defined.

src/transports/winhttp.c

@@ -48,6 +48,10 @@
 # define WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_2 0x00000800
 #endif
 
+#ifndef DWORD_MAX
+# define DWORD_MAX 0xffffffff
+#endif
+
 static const char *prefix_https = "https://";
 static const char *upload_pack_service = "upload-pack";
 static const char *upload_pack_ls_service_url = "/info/refs?service=git-upload-pack";