update copyright headers for files changed this year

apps/app_chanspy.c

apps/app_mixmonitor.c

apps/app_stack.c

ast_expr2.fl

channel.c

channels/chan_iax2.c

channels/chan_sip.c

file.c

image.c

include/asterisk/chanspy.h

include/asterisk/image.h

include/asterisk/rtp.h

manager.c

pbx.c

rtp.c