generic_serial.h: kill incorrect gs_debug reference

generic_serial.h contained an incorrect extern reference to the static variable gs_debug (Benoit Boissinot reported that gcc 4.0 rejects this).

---

This patch was already sent on:
- 21 Jan 2005

@@ -93,6 +93,4 @@
 int  gs_getserial(struct gs_port *port, struct serial_struct __user *sp);
 void gs_got_break(struct gs_port *port);
 
-extern int gs_debug;
-
 #endif