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>

lib/Kconfig

@@ -451,7 +451,8 @@ config MPILIB
 
 config SIGNATURE
 	tristate
-	depends on KEYS && CRYPTO
+	depends on KEYS
+	select CRYPTO
 	select CRYPTO_SHA1
 	select MPILIB
 	help