graphics/py-pygraphviz: fix build with clang 15

During an exp-run for llvm 15 (see bug 265425), it turned out that
graphics/py-pygraphviz failed to build with clang 15:

  pygraphviz/graphviz_wrap.c:3047:12: error: incompatible pointer to integer conversion returning 'Agsym_t *' (aka 'struct Agsym_s *') from a function with result type 'int' [-Wint-conversion]
      return agattr(g, kind, name, val);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

In graphviz.i and the swig-generated graphviz_wrap.c, the agattr_label()
function is incorrectly labeled as returning 'int', while it should
return 'Agsym_t *' instead. Upstream fixed this as part of a much bigger
commit [1], and they also regenerated the swig wrapper.

For our case, it is simpler to patch the wrapper instead, as otherwise
you would have to add swig as a build-time dependency.

[1] https://github.com/pygraphviz/pygraphviz/commit/79fdee178785620b90241c70a0efab9dd57e3c86

PR:		268278
Approved by:	portmgr (tcberner)
MFH:		2022Q4