add a return type to test function used by xcompile

This fixes a minor bug that could cause testcc to fail unexpectedly.

Change-Id: Ib75d343104b6937682c05acf5232596aac83f105
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/2068
Tested-by: build bot (Jenkins)

util/xcompile/xcompile

@@ -40,7 +40,7 @@ testcc() {
 	local tmp_c="$TMPFILE.c"
 	local tmp_o="$TMPFILE.o"
 	rm -f "$tmp_c" "$tmp_o"
-	echo "_start(void) {}" >"$tmp_c"
+	echo "void _start(void) {}" >"$tmp_c"
 	"$1" -nostdlib -Werror $2 "$tmp_c" -o "$tmp_o" >/dev/null 2>&1
 }