ARM: OMAP2+: twl-common: Fix compile time error when omap-twl4030 audio is not enabled

Fixes:
CC      arch/arm/mach-omap2/twl-common.o arch/arm/mach-omap2/twl-common.c:562: error: conflicting types for ‘omap_twl4030_audio_init’
arch/arm/mach-omap2/twl-common.h:62: error: previous declaration of ‘omap_twl4030_audio_init’ was here
make[1]: *** [arch/arm/mach-omap2/twl-common.o] Error 1
make: *** [arch/arm/mach-omap2] Error 2

When building the kernel with !SND_OMAP_SOC_OMAP_TWL4030

Reported-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>