xfconf-query: Fix GOptionContext memory leak
Leak report:
988 (88 direct, 900 indirect) bytes in 1 blocks are definitely lost in loss record 1,179 of 1,190
at 0x484BC13: calloc (vg_replace_malloc.c:1675)
by 0x4ADE2C2: g_malloc0 (gmem.c:133)
by 0x4ADE318: g_option_context_new (goption.c:208)
by 0x10BD51: main (main.c:244)
Fixes one leak from #41 (closed)