systemd: fix build with libgcrypt enabled

When gcrypt support is enabled, we need to tell systemd where
libgpg-error is installed.

Fixes:

  http://autobuild.buildroot.org/results/1fd6193ff33250565c95e53614f5aa7fc6f4581c/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>