libpayload: Fix prototype warnings in PDCurses

This fixes the following PDCurses warnings:

    CC         curses/pdcurses-backend/pdcsetsc.libcurses.o
curses/pdcurses-backend/pdcsetsc.c: In function 'PDC_curs_set':
curses/pdcurses-backend/pdcsetsc.c:17:9: warning: implicit declaration of function 'serial_cursor_enable' [-Wimplicit-function-declaration]
curses/pdcurses-backend/pdcsetsc.c:22:9: warning: implicit declaration of function 'video_console_cursor_enable' [-Wimplicit-function-declaration]
    CC         curses/pdcurses-backend/pdcutil.libcurses.o
curses/pdcurses-backend/pdcutil.c:30:6: warning: no previous prototype for 'curses_enable_serial' [-Wmissing-prototypes]
curses/pdcurses-backend/pdcutil.c:35:6: warning: no previous prototype for 'curses_enable_vga' [-Wmissing-prototypes]
curses/pdcurses-backend/pdcutil.c:40:5: warning: function declaration isn't a prototype [-Wstrict-prototypes]
curses/pdcurses-backend/pdcutil.c:45:5: warning: function declaration isn't a prototype [-Wstrict-prototypes]

Change-Id: If0d4d475d3006f1a77f67ec46c6bdf4ee2906981
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/2908
Reviewed-by: Dave Frodin <dave.frodin@se-eng.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

payloads/libpayload/curses/pdcurses-backend/pdcsetsc.c

@@ -1,6 +1,7 @@
 /* Public Domain Curses */
 /* This file is BSD licensed, Copyright 2011 secunet AG */
 
+#include <libpayload.h>
 #include "lppdc.h"
 
 int PDC_curs_set(int visibility)