build(meson): add support for libomemo-c as OMEMO backend

Add the ability to choose between libsignal-protocol-c (default)
and libomemo-c when building with OMEMO support enabled in Meson.

Close: https://github.com/profanity-im/profanity/pull/2020

src/omemo/store.c

@@ -7,7 +7,12 @@
  * SPDX-License-Identifier: GPL-3.0-or-later WITH OpenSSL-exception
  */
 #include <glib.h>
+
+#ifdef HAVE_LIBOMEMO_C
+#include <omemo/signal_protocol.h>
+#else
 #include <signal/signal_protocol.h>
+#endif
 
 #include "config.h"
 #include "log.h"