certs: use if_changed to re-generate the key when the key type is changed

If the key type of the existing signing key does not match to
CONFIG_MODULE_SIG_KEY_TYPE_*, the Makefile removes it so that it is
re-generated.

Use if_changed so that the key is re-generated when the key type is
changed (that is, the openssl command line is changed).

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>