Export variables needed for system console initialization.

Do this in a separate file because pcvt_hdr.h contains too many definitions
which can cause namespace trouble. (eg, a structure element "m_len")

sys/arch/i386/isa/pcvt/pcvt_cons.h

@@ -0,0 +1,4 @@
+int pccngetc __P((dev_t));
+void pccnputc __P((dev_t, int));
+void pccnpollc __P((dev_t, int));
+void pccninit __P((struct consdev *));

sys/arch/i386/isa/pcvt/pcvt_drv.c

@@ -88,8 +88,8 @@
 #if NVT > 0
 
 #define EXTERN			/* allocate mem */
-
 #include "pcvt_hdr.h"		/* global include */
+#include "pcvt_cons.h"		/* NetBSD only */
 
 #ifdef NOTDEF
 unsigned	__debug = 0; /*0xffe */;