utils/cache_gc: fix debug prints

The difference is on "negative" char values, in which case
the prints were way too long.

utils/cache_gc/db.c

@@ -128,7 +128,7 @@ void debug_printbin(const char *str, unsigned int len)
 		if (isprint(c))
 			putchar(c);
 		else
-			printf("`%02x`", c);
+			printf("`%02hhx`", c);
 	}
 	putchar('"');
 }