gnu: elogind: Add '-L' flag for libcap in libelogind.la.

This fix allows us to revert this series of commits:

  1b97e926 * gnu: gdm: Add missing input.
  de0c89f1 * gnu: colord: Add missing input.
  7e38ba65 * gnu: accountsservice: Add missing input.
  166bc1b2 * gnu: network-manager: Add missing input.
  daae2089 * gnu: modem-manager: Add missing input.
  1f919459 * gnu: udisks: Add missing input.
  744e9d07 * gnu: mutter: Add missing input.
  0cce140d * gnu: gnome-session: Add missing input.
  77c2476a * gnu: polkit: Add missing input.
  124ac301 * gnu: weston: Add missing input.
  472c2223 * gnu: kmscon: Add missing input.

* gnu/packages/freedesktop.scm (elogind)[arguments]: Add
'add-libcap-to-search-path' phase.
(weston)[inputs]: Remove LIBCAP.
(udisks): Likewise.
(accountsservice): Likewise.
(modem-manager): Likewise.
* gnu/packages/gnome.scm (colord): Likewise
(gnome-session): Likewise.
(mutter): Likewise.
(network-manager): Likewise.
(gdm): Likewise.
* gnu/packages/polkit.scm (polkit): Likewise.
* gnu/packages/terminals.scm (kmscon): Likewise.