siglent-sds: Drop currently unused <rpc/rpc.h> #include.

This causes some compile errors on platforms where RPC is not available, e.g.

  siglent-sds/api.c:25:21: fatal error: rpc/rpc.h: No such file or directory

Surrounding the #include (and later code) with "#if HAVE_RPC" should avoid
these compile errors, but since it's not used at all for now, drop it.

src/hardware/siglent-sds/api.c

@@ -22,7 +22,6 @@
 #include <fcntl.h>
 #include <glib.h>
 #include <math.h>
-#include <rpc/rpc.h>
 #include <stdlib.h>
 #include <string.h>
 #include <strings.h>