As "get_persconffile_path()" doesn't return a "const char *", neither

As "get_persconffile_path()" doesn't return a "const char *", neither
does "get_plugins_pers_dir()" - and "get_plugins_global_dir()" doesn't
return one either.  Both of them return mallocated data, and making them
return a "const char *" just causes compiler whining when you try to
free them.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@12883 f5534014-38df-0310-8fa8-9805f1628bb7