scripts/ipkg-build: fix a build regression introduced in r46360 (fixes #20082)

go to the $TOPDIR directory before trying to extract a git/svn timestamp

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46383 3c298f89-4303-0410-b956-a3cf2f4a3e73

scripts/ipkg-build

@@ -19,7 +19,7 @@ export GZIP="-n"
 
 # look up date of last commit
 if [ -d "$TOPDIR/.git" ]; then
-	TIMESTAMP=$($GIT log -1 -s --format=%ci)
+	TIMESTAMP=$(cd $TOPDIR; $GIT log -1 -s --format=%ci)
 elif [ -d "$TOPDIR/.svn" ]; then
 	TIMESTAMP=$($SVN info "$TOPDIR" | sed -n "s/^Last Changed Date: \(.*\)/\1/p")
 else