makepkg: fix double check on same path

Commit 142225fd typo-ed this one, as it should look like the rest of the
checks similar to it.

Signed-off-by: Dan McGee <dan@archlinux.org>

scripts/makepkg.sh.in

@@ -578,7 +578,7 @@ check_checksums() {
 				echo -n "    $file ... " >&2
 
 				if [ ! -f "$file" ] ; then
-					if [ ! -f "$file" ] ; then
+					if [ ! -f "$SRCDEST/$file" ] ; then
 						echo "$(gettext "NOT FOUND")" >&2
 						errors=1
 						found=0