Add a tag name to hamlib_port_t.

I am making a Haskell binding to hamlib and this anonymous struct was
creating some issues for me. I am not a C-coder by day, but I think this
is harmless to add here.

Signed-off-by: Ricky Elrod <ricky@elrod.me>

include/hamlib/rig.h

@@ -1335,7 +1335,7 @@ struct rig_caps {
  *
  * Of course, looks like OO painstakingly programmed in C, sigh.
  */
-typedef struct {
+typedef struct hamlib_port {
   union {
 	rig_port_t rig;		/*!< Communication port type */
 	ptt_type_t ptt;		/*!< PTT port type */