Fix a sign mismatch warning in the physmem code

Make sure both sides of a comparison are unsigned. As the values being
compared are size_t make the the value in the for loop size_t too.

Sponsored by:	The FreeBSD Foundation

sys/kern/subr_physmem.c

@@ -95,7 +95,8 @@ long Maxmem;
 static void
 physmem_dump_tables(int (*prfunc)(const char *, ...))
 {
-	int flags, i;
+	size_t i;
+	int flags;
 	uintmax_t addr, size;
 	const unsigned int mbyte = 1024 * 1024;