Staging: android: binder: Make task_get_unused_fd_flags function static

Silence the following warning:
drivers/staging/android/binder.c:368:5: warning:
symbol 'task_get_unused_fd_flags' was not declared. Should it be static?

Cc: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>