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/crypto.c

@@ -9,8 +9,13 @@
 #include "config.h"
 
 #include <assert.h>
+#ifdef HAVE_LIBOMEMO_C
+#include <omemo/signal_protocol.h>
+#include <omemo/signal_protocol_types.h>
+#else
 #include <signal/signal_protocol.h>
 #include <signal/signal_protocol_types.h>
+#endif
 
 #include "log.h"
 #include "omemo/omemo.h"