scripts: avoid matching 'char **' as string for systemtap

scripts: avoid matching 'char **' as string for systemtap

When a probe argument is declared "char *" we reference the userspace
string value using 'user_string(...)' for systemtap.

Unfortunately our code generator also matches on args declared "char **"
and generates bogus code

   *cert = user_string($arg4);

which is a syntax error for systemtap.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>