* Expose rgb_matrix_update_pwm_buffers to be available in keymaps
Exposing this API enables the examples in the keyboard shutdown/reboot
documentation to compile instead of calling `rgb_matrix_driver.flush`
directly.
* Remove extraneous rgb_matrix_update_pwm_buffers prototype declarations
Now that `rgb_matrix_update_pwm_buffers` is public, there isn't the need
for local prototype declarations.
