contrib/plugins/howvec: Fix string format

This fixes on Darwin:

  plugins/howvec.c:186:40: warning: format specifies type 'long' but the argument has type 'uint64_t' (aka 'unsigned long long') [-Wformat]
                                         class->count);
                                         ^~~~~~~~~~~~
  plugins/howvec.c:213:36: warning: format specifies type 'long' but the argument has type 'uint64_t' (aka 'unsigned long long') [-Wformat]
                                     rec->count,
                                     ^~~~~~~~~~

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20230907105004.88600-4-philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>