ANDROID: KVM: arm64: Fix link with CONFIG_MODULES=n

The recently introduced support for pKVM modules has clearly lacked a
bit of testing with CONFIG_MODULES=n.

Make sure to not access __pkvm_modules_enabled when CONFIG_MODULES=n to
fix the link.

Bug: 244543039
Bug: 244373730
Reported-by: kernel test robot <lkp@intel.com>
Change-Id: I65c6ad7c07c2812b85dcd435b24b3444c0ce7f1e
Signed-off-by: Quentin Perret <qperret@google.com>