sparc32: fix sparse warnings in windows.c

Fix following warnings:
windows.c:16:6: warning: symbol 'flush_user_windows' was not declared. Should it be static?
windows.c:109:6: warning: symbol 'try_to_clear_window_buffer' was not declared. Should it be static?

Add missing include of cacheflush.h + add declaration of try_to_clear_window_buffer in kernel.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>