Update random.h

While I'm making an argument for better consistency, I might as well be self-consistent.

src/random.h

@@ -20,7 +20,7 @@ void RandAddSeedPerfmon();
 /**
  * Functions to gather random data via the OpenSSL PRNG
  */
-void GetRandBytes(unsigned char* buf, int num);
+void GetRandBytes(unsigned char* buf, size_t num);
 uint64_t GetRand(uint64_t nMax);
 int GetRandInt(int nMax);
 uint256 GetRandHash();