We do not have syscall.h.  We do have sys/syscall.h instead.
However, the syscall.h include is apparently not needed anyway.

--- libotr-4.1.1/tests/regression/client/client.c.orig
+++ libotr-4.1.1/tests/regression/client/client.c
@@ -24,7 +24,6 @@
 #include <pthread.h>
 #include <signal.h>
 #include <stdlib.h>
-#include <syscall.h>
 #include <sys/epoll.h>
 #include <sys/types.h>
 #include <sys/un.h>