Fix windows-gnu build.

We can't define `HIDDEN` to be an empty string, the way `HIDDEN`
is defined & used.

Author: briansmith

crypto/curve25519/asm/x25519-asm-x86_64.S

@@ -29,7 +29,7 @@
 #define HIDDEN .private_extern
 #elif defined(__MINGW32__) || defined(__MINGW64__)
 #define C_ABI(x) x
-#define HIDDEN
+#define HIDDEN .globl
 #else
 #define C_ABI(x) x
 #define HIDDEN .hidden