toggle_interruptibility() is used only by same file, it should be static.

Fixed following sparse warning :

  arch/x86/kvm/x86_emulate.c:1364:6: warning: symbol 'toggle_interruptibility' was not declared. Should it be static?
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Avi Kivity <avi@redhat.com>