When libattr is selected, libtalloc links to this shared library and
the build fails with:
"Package libtalloc is missing dependencies for the following libraries:
libattr.so.1"

This commit adds libattr dependency, so that libtalloc systematically
links to this library.

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>
[Lucile: replace attr with libattr, edit commit message]
Signed-off-by: Lucile Quirion <lucile.quirion@savoirfairelinux.com>

libtalloc is a memory allocator used in Samba4 and freeradius-server
version 3.0.x and higher.

Signed-off-by: Lucile Quirion <lucile.quirion@savoirfairelinux.com>