mime: Fix GOptionContext leak

Leak report

Direct leak of 88 byte(s) in 1 object(s) allocated from:
    #0 0x5999c997bb69 in calloc (/usr/asan/bin/xfce4-mime-helper+0x16db69)
    #1 0x770d1add50ca in g_malloc0 glib/gmem.c:133:13
    #2 0x770d1addf873 in g_option_context_new glib/goption.c:208:13
    #3 0x5999c99d0178 in main xfce4-settings/dialogs/mime-settings/xfce-mime-helper-main.c:125:13

Steps to reproduce

• xfce4-mime-helper --query

Testing

• No issues were detected with LSan, ASan, or Valgrind when testing the patch