ASoC: max98357a: Fix missing include

This fixes the following compilation errors:

  sound/soc/codecs/max98357a.c: In function ‘max98357a_daiops_trigger’:
  sound/soc/codecs/max98357a.c:30:3: error: implicit declaration of function ‘gpiod_set_value’ [-Werror=implicit-function-declaration]
  sound/soc/codecs/max98357a.c: In function ‘max98357a_codec_probe’:
  sound/soc/codecs/max98357a.c:55:2: error: implicit declaration of function ‘devm_gpiod_get’ [-Werror=implicit-function-declaration]
  sound/soc/codecs/max98357a.c:61:2: error: implicit declaration of function ‘gpiod_direction_output’ [-Werror=implicit-function-declaration]
  cc1: some warnings being treated as errors

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Kenneth Westfield <kwestfie@codeaurora.org>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>