Include cstddef to avoid the following build failure with gcc 11:

In file included from /tmp/instance-3/output-1/build/pistache-f2f5a50f/src/../include/pistache/async.h:10,
                 from /tmp/instance-3/output-1/build/pistache-f2f5a50f/src/../include/pistache/client.h:9,
                 from /tmp/instance-3/output-1/build/pistache-f2f5a50f/src/client/client.cc:7:
/tmp/instance-3/output-1/build/pistache-f2f5a50f/src/../include/pistache/typeid.h:26:12: error: expected type-specifier before 'size_t'
   26 |   operator size_t() const { return reinterpret_cast<size_t>(id_); }
      |            ^~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/2443559df8c2357476e4cbdbebb08280cbb80a3bSigned-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>