find-libdeps: fix extraction of soname

libperl.so results in soname="libperl.so.so" which is wrong.

This returns the correct string: "libperl.so"

Fix-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Florian Pritz <bluewind@xinu.at>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>