srd.c: Fix a compiler warning with gcc 8.

  srd.c: In function ‘srd_searchpaths_get’:
  srd.c:399:40: warning: cast between incompatible function types from ‘gchar * (*)(const gchar *)’ {aka ‘char * (*)(const char *)’} to ‘void * (*)(const void *, void *)’ [-Wcast-function-type]
    return g_slist_copy_deep(searchpaths, (GCopyFunc)g_strdup, NULL);
                                          ^

Upstream glib issue / documentation change:
https://gitlab.gnome.org/GNOME/glib/issues/1492
https://gitlab.gnome.org/pwithnall/glib/commit/e81f4c2acea5ada6ae989426e462613f7c612cac