gstmirsink.c: adding missing set_input_cb_and_ctx platform-api call

Fixes the touch crash when using playbin.

Signed-off-by: Ricardo Salveti de Araujo <ricardo.salveti@canonical.com>
Merge pull request #5 from rsalveti/fixes

gstmirsink.c: adding missing set_input_cb_and_ctx platform-api call