elfutils: remove argp-standalone dependency

It is required only when building elfutils binaries (readelf, ld ...)
but not the library itself.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@35558 3c298f89-4303-0410-b956-a3cf2f4a3e73