backlight: corgi_lcd: fix missing prototype

The corgi_lcd_limit_intensity() function is called from platform
and defined in a driver, but the driver does not see the declaration:

drivers/video/backlight/corgi_lcd.c:434:6: error: no previous prototype for 'corgi_lcd_limit_intensity' [-Werror=missing-prototypes]
434 | void corgi_lcd_limit_intensity(int limit)

Move the prototype into a header that can be included from both
sides to shut up the warning.

Reviewed-by: Daniel Thompson <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

arch/arm/mach-pxa/sharpsl_pm.h

···
 #define MAX1111_ACIN_VOLT   6u
 int sharpsl_pm_pxa_read_max1111(int channel);
 
-void corgi_lcd_limit_intensity(int limit);
 #endif

arch/arm/mach-pxa/spitz_pm.c

···
 #include <linux/interrupt.h>
 #include <linux/platform_device.h>
 #include <linux/apm-emulation.h>
+#include <linux/spi/corgi_lcd.h>
 
 #include <asm/irq.h>
 #include <asm/mach-types.h>

include/linux/spi/corgi_lcd.h

···
 	void (*kick_battery)(void);
 };
 
+void corgi_lcd_limit_intensity(int limit);
+
 #endif /* __LINUX_SPI_CORGI_LCD_H */