Add -Werror to xcompile's testcc

If -Werror is not specified, tests for certain compiler flags
will emit a warning, which makes the build break since we compile
with -Werror.

Change-Id: I7be56530ff9f94e5500bad226c83e47145a808d7
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/336
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

util/xcompile/xcompile

@@ -21,7 +21,7 @@
 testcc()
 {
 	echo "_start(void) {}" > .$$$$.c
-	$1 -nostdlib $2 .$$$$.c -o .$$$$.tmp 2>/dev/null >/dev/null
+	$1 -nostdlib -Werror $2 .$$$$.c -o .$$$$.tmp 2>/dev/null >/dev/null
 	ret=$?
 	rm -f .$$$$.c .$$$$.tmp
 	return $ret