x86/ima: add missing include

As reported by 0-DAY kernel test infrastructure:
   arch/x86//kernel/ima_arch.c: In function 'arch_get_ima_policy':
>> arch/x86//kernel/ima_arch.c:78:4: error: implicit declaration of
function 'set_module_sig_enforced' [-Werror=implicit-function-declaration]

Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>