[mucurses] Fix erroneous __nonnull attribute

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/curses.h

 extern int wclrtobot ( WINDOW * ) __nonnull;
 extern int wclrtoeol ( WINDOW * ) __nonnull;
 extern void wcursyncup ( WINDOW * );
-extern int wcolour_set ( WINDOW *, short, void * ) __nonnull;
+extern int wcolour_set ( WINDOW *, short, void * )
+	__attribute__ (( nonnull (1)));
 #define wcolor_set(w,s,v) wcolour_set((w),(s),(v))
 extern int wdelch ( WINDOW * ) __nonnull;
 extern int wdeleteln ( WINDOW * ) __nonnull;