ima: ensure IMA_APPRAISE_MODSIG has necessary dependencies

IMA_APPRAISE_MODSIG is used for verifying the integrity of both kernel
and modules. Enabling IMA_APPRAISE_MODSIG without MODULES causes a build
break.

Ensure the build time kernel signing key is only generated if both
IMA_APPRAISE_MODSIG and MODULES are enabled.

Fixes: 0165f4ca ("ima: enable signing of modules with build time generated key")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>