json_util: define `strtoll` as _strtoi64 for MSVC

Got the idea from this blog post:
  http://www.enchantedage.com/node/231

Simple & concise stuff :)

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

json_util.c

@@ -39,6 +39,7 @@
 #endif /* HAVE_UNISTD_H */
 
 #ifdef WIN32
+# define strtoll _strtoi64
 # define WIN32_LEAN_AND_MEAN
 # include <windows.h>
 # include <io.h>