ASoC: rockchip: pdm: select CONFIG_RATIONAL

Without this, we get a link error:

sound/soc/rockchip/rockchip_pdm.o: In function `rockchip_pdm_hw_params':
rockchip_pdm.c:(.text+0x754): undefined reference to `rational_best_approximation'

Fixes: 624e8e00 ("ASoC: rockchip: pdm: fixup pdm fractional div")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/rockchip/Kconfig

@@ -19,6 +19,7 @@ config SND_SOC_ROCKCHIP_PDM
 	tristate "Rockchip PDM Controller Driver"
 	depends on CLKDEV_LOOKUP && SND_SOC_ROCKCHIP
 	select SND_SOC_GENERIC_DMAENGINE_PCM
+	select RATIONAL
 	help
 	  Say Y or M if you want to add support for PDM driver for
 	  Rockchip PDM Controller. The Controller supports up to maximum of