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: Simon Glass <sjg@chromium.org>
diff --git a/lib/crypto/Kconfig b/lib/crypto/Kconfig
index 509bc28..1c04a7e 100644
--- a/lib/crypto/Kconfig
+++ b/lib/crypto/Kconfig
@@ -1,5 +1,6 @@
 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