[winpr] add WINPR_ATTR_FORMAT_ARG

this macro declares the function argument (pos,arg) to be a printf
style format string and enables compiler checks for that.

server/Sample/sfreerdp.c

@@ -1257,6 +1257,7 @@ static const struct
 	const char skey[6];
 } options = { "--pcap=", "--fast", "--port=", "--local-only", "--cert=", "--key=" };
 
+WINPR_ATTR_FORMAT_ARG(2, 0)
 static void print_entry(FILE* fp, const char* fmt, const char* what, size_t size)
 {
 	char buffer[32] = { 0 };