systemd: depend on libgcrypt

systemd: depend on libgcrypt

Although libgcrypt support is optional, its development files are
are always required to expand AM_PATH_LIBGCRYPT.  Build might fails else
with

| libtoolize: copying file `m4/lt~obsolete.m4'
| configure.ac:446: warning: macro 'AM_PATH_LIBGCRYPT' not found in library
| ...
| configure.ac:446: error: possibly undefined macro: AM_PATH_LIBGCRYPT
| autoreconf: .../autoconf failed with exit status: 1
| + bbfatal 'autoreconf execution failed.'
| + echo 'ERROR: autoreconf execution failed.'

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>