backlight: ld9040: Staticize local variable gamma_table

Fix the following sparse warnings:

  drivers/video/backlight/ld9040_gamma.h:172:3: warning: symbol 'gamma_table' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>