Make ASYMMETRIC_KEY_TYPE depend on FIT_SIGNATURE

Add this dependency to avoid a build error if FIT_SIGNATURE is not
enabled.
Signed-off-by: NSimon Glass <sjg@chromium.org>

lib/crypto/Kconfig

 menuconfig ASYMMETRIC_KEY_TYPE
 	bool "Asymmetric (public-key cryptographic) key Support"
+	depends on FIT_SIGNATURE
 	help
 	  This option provides support for a key type that holds the data for
 	  the asymmetric keys used for public key cryptographic operations such