arch: x86: add missing semicolon for z_x86_kpti_is_access_ok()

The function prototype / extern declaration for
z_x86_kpti_is_access_ok() in x86_mmu.h was missing a semicolon.

Add it to avoid being surprised by compile errors in certain
circumstances.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>