fbdev: hitfb: Declare hitfb_blank() as static

Fixes the following warnings:

../drivers/video/fbdev/hitfb.c:186:5: warning: no previous prototype for 'hitfb_blank' [-Wmissing-prototypes]
  186 | int hitfb_blank(int blank_mode, struct fb_info *info)
      |     ^~~~~~~~~~~

../drivers/video/fbdev/hitfb.c:186:5: warning: symbol 'hitfb_blank' was not declared. Should it be static?

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Helge Deller <deller@gmx.de>