libmakepkg/srcinfo.sh: fix missing extglob in srcinfo_write_attr
We use an extended glob here, but were relying on having it globally set in makepkg. This causes it to fail when used in scripts.
Since scripts using libmakepkg may not want extglob to be set, save and restore the environment while explicitly setting extglob only where we need it.
Signed-off-by: Levente Polyak anthraxx@archlinux.org