package/argp-standalone: is useless for (e)glibc toolchains

According to Thomas's remark [1], (e)glibc toolchains provide it's
own argp.h version.
Also, it appears that argp.h from glibc is more up to date should
be preferred to argp-standalone one.

[1] http://lists.busybox.net/pipermail/buildroot/2015-May/127511.html



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>