Fix assumed return type of MSVC

Wenfeng CAI

2018-11-13 a0b74b09a5824d50613e3b3a5590e7a9ed515716

Fix assumed return type of MSVC

With unbase64() undeclared, MSVS will assume the return type is int
while it should be a pointer. int is 32 bit and pointer is 64 bit on
Win64 platform. Without this patch, it will fail in Win64 environment.

 src/library/os/os.c

@@ -88,6 +88,8 @@
#include <Windows.h>
#pragma comment(lib, "IPHLPAPI.lib")

unsigned char* unbase64(const char* ascii, int len, int *flen);

static void printHash(HCRYPTHASH* hHash) {
    BYTE *pbHash;
    DWORD dwHashLen;