3rd_party/libsrp6a: Add windows.h because Windows is weird

3rd_party/libsrp6a-sha512/t_truerand.c

diff --git a/3rd_party/libsrp6a-sha512/t_truerand.c b/3rd_party/libsrp6a-sha512/t_truerand.c
index 2617b5e..4a4c3d2 100644
--- a/3rd_party/libsrp6a-sha512/t_truerand.c
+++ b/3rd_party/libsrp6a-sha512/t_truerand.c
@@ -71,6 +71,7 @@
 
 # else /* !CRYPTOLIB && WIN32 */
 
+#include <windows.h>
 #include <wtypes.h>
 #include <winbase.h>
 #include <windef.h>