spl: fit: select SPL_CRYPTO_SUPPORT for SPL_FIT_SIGNATURE

Verifying FIT images obviously needs the rsa parts of crypto
support and while main uboot always compiles crypto support,
it's optional for SPL and we should thus select the necessary
option to not end up in compile errors like:

    u-boot/lib/rsa/rsa-verify.c:328: undefined reference to `rsa_mod_exp'

So select SPL_CRYPTO_SUPPORT in SPL_FIT_SIGNATURE.
Signed-off-by: NHeiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>

Kconfig

@@ -509,6 +509,7 @@ config SPL_FIT_SIGNATURE
 	bool "Enable signature verification of FIT firmware within SPL"
 	depends on SPL_DM
 	select SPL_FIT
+	select SPL_CRYPTO_SUPPORT
 	select SPL_HASH_SUPPORT
 	select SPL_RSA
 	select SPL_RSA_VERIFY