kvm: Delete prototype for non-existent function kvm_check_iopl

The prototype for kvm_check_iopl appeared in commit
f850e2e6 ("KVM: x86 emulator: Check IOPL
level during io instruction emulation"), but the function never actually
existed.  Remove the prototype.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Gleb Natapov <gleb@redhat.com>