kvm: selftests: use -pthread instead of -lpthread

I run into the following error

testing/selftests/kvm/dirty_log_test.c:285: undefined reference to `pthread_create'
testing/selftests/kvm/dirty_log_test.c:297: undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status

my gcc version is gcc version 4.8.4
"-pthread" would work everywhere

Signed-off-by: Lei Yang <Lei.Yang@windriver.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>