kvm: x86: make a function in x86.c static to avoid compiling warning

kvm_emulate_wbinvd_noskip is only used in x86.c, and should be
static to avoid compiling warning when with -Wmissing-prototypes
option.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>