FreeBSD needs libutil.h for forkpty()

The header is not available everywhere, so only include it if it
is present.  (The extra include should be harmless on Linux by
inspection.)

src/frontend/mosh-server.cc

@@ -20,6 +20,9 @@
 
 #include <errno.h>
 #include <locale.h>
+#ifdef HAVE_LIBUTIL_H
+#include <libutil.h>
+#endif
 #include <string.h>
 #include <termios.h>
 #include <unistd.h>