digsig: make crypto builtin if digsig selected as builtin

When SIGNATURE=y but depends on CRYPTO=m, it selects MPILIB as module
producing build break. This patch makes digsig to select crypto for
correcting dependency.

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: David Howells <dhowells@redhat.com>