kselftest: ksft_test_num return type should be unsigned

Fixes a compiler warning:

In file included from sync_test.c:37:
../kselftest.h: In function ‘ksft_print_cnts’:
../kselftest.h:78:16: warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Wsign-compare]
  if (ksft_plan != ksft_test_num())
                ^~

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>