selftests: kvm: fix build with glibc >= 2.30

Glibc-2.30 gained gettid() wrapper, selftests fail to compile:

lib/assert.c:58:14: error: static declaration of ‘gettid’ follows non-static declaration
   58 | static pid_t gettid(void)
      |              ^~~~~~
In file included from /usr/include/unistd.h:1170,
                 from include/test_util.h:18,
                 from lib/assert.c:10:
/usr/include/bits/unistd_ext.h:34:16: note: previous declaration of ‘gettid’ was here
   34 | extern __pid_t gettid (void) __THROW;
      |                ^~~~~~

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>