[media] arm: omap2: rx51-peripherals: fix build warning

commit 68a3c0433077 ([media] ARM: OMAP2: RX-51: update
si4713 platform data) updated board-rx51-peripherals.c
so that si4713 could be easily used on DT boot, but
it ended up introducing a build warning whenever
si4713 isn't enabled.

This patches fixes that warning:

arch/arm/mach-omap2/board-rx51-peripherals.c:1000:36: warning: \
‘rx51_si4713_platform_data’ defined but not used [-Wunused-variable]
static struct si4713_platform_data rx51_si4713_platform_data = {

Cc: Sebastian Reichel <[e-mail]>
Cc: Tony Lindgren <[e-mail]>
Cc: Hans Verkuil <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

arch/arm/mach-omap2/board-rx51-peripherals.c

···
 	.gpio_reset = 60,
 };
 
+#if IS_ENABLED(CONFIG_I2C_SI4713) && IS_ENABLED(CONFIG_PLATFORM_SI4713)
 static struct si4713_platform_data rx51_si4713_platform_data = {
 	.is_platform_device = true
 };
+#endif
 
 static struct i2c_board_info __initdata rx51_peripherals_i2c_board_info_2[] = {
 #if IS_ENABLED(CONFIG_I2C_SI4713) && IS_ENABLED(CONFIG_PLATFORM_SI4713)