ARM: S3C24XX: Make 'clk_msysclk' static

Fixes the following warning:
arch/arm/mach-s3c24xx/common-s3c2443.c:135:19: warning:
symbol 'clk_msysclk' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>