Add omemo_close function

We call omemo_init() when starting profanity and should have an
omemo_close() at exit.

For now we free the fingerprint autocompleter in there.

Fixes valgrind:
```
==13226== 24 bytes in 1 blocks are definitely lost in loss record 2,855
of 6,958
==13226==    at 0x483677F: malloc (in
/usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==13226==    by 0x48AD39: autocomplete_new (autocomplete.c:57)
==13226==    by 0x4AB89F: omemo_init (omemo.c:127)
==13226==    by 0x42C283: _init (profanity.c:206)
==13226==    by 0x42BFF3: prof_run (profanity.c:98)
==13226==    by 0x4B25E6: main (main.c:172)
```

Regards https://github.com/profanity-im/profanity/issues/1131

src/omemo/omemo.c

@@ -127,6 +127,15 @@ omemo_init(void)
     omemo_ctx.fingerprint_ac = autocomplete_new();
 }
 
+void
+omemo_close(void)
+{
+    if (omemo_ctx.fingerprint_ac) {
+        autocomplete_free(omemo_ctx.fingerprint_ac);
+        omemo_ctx.fingerprint_ac = NULL;
+    }
+}
+
 void
 omemo_on_connect(ProfAccount *account)
 {