bin/xbps-checkvers: store character returned by fgetc in int

The character has to be checked for EOF (-1), otherwise with
architectures that use unsigned char by default this breaks at
compile and runtime.

https://github.com/void-linux/xbps/issues/123

bin/xbps-checkvers/main.c

@@ -219,7 +219,7 @@ rcv_sh_substitute(rcv_t *rcv, const char *str, size_t len)
 				p++;
 				if (*p == '(') {
 					FILE *fp;
-					char c;
+					int c;
 					for (ref = ++p; *p && p < str+len && *p != ')'; p++)
 						;
 					if (*p != ')')