Include config.h in omemo files

So that off_t is defined in all files (so far http_upload.c and
http_download.c too) the same way.

Fix https://github.com/profanity-im/profanity/issues/1512

Thanks @pasis for finding the solution!

src/omemo/omemo.c

@@ -33,6 +33,8 @@
  * source files in the program, then also delete it here.
  *
  */
+#include "config.h"
+
 #include <sys/time.h>
 #include <sys/stat.h>