fbdev/sh7760fb: Depend on FB=y

Fix linker error if FB=m about missing fb_io_read and fb_io_write. The
linker's error message suggests that this config setting has already
been broken for other symbols.

  All errors (new ones prefixed by >>):

     sh4-linux-ld: drivers/video/fbdev/sh7760fb.o: in function `sh7760fb_probe':
     sh7760fb.c:(.text+0x374): undefined reference to `framebuffer_alloc'
     sh4-linux-ld: sh7760fb.c:(.text+0x394): undefined reference to `fb_videomode_to_var'
     sh4-linux-ld: sh7760fb.c:(.text+0x39c): undefined reference to `fb_alloc_cmap'
     sh4-linux-ld: sh7760fb.c:(.text+0x3a4): undefined reference to `register_framebuffer'
     sh4-linux-ld: sh7760fb.c:(.text+0x3ac): undefined reference to `fb_dealloc_cmap'
     sh4-linux-ld: sh7760fb.c:(.text+0x434): undefined reference to `framebuffer_release'
     sh4-linux-ld: drivers/video/fbdev/sh7760fb.o: in function `sh7760fb_remove':
     sh7760fb.c:(.text+0x800): undefined reference to `unregister_framebuffer'
     sh4-linux-ld: sh7760fb.c:(.text+0x804): undefined reference to `fb_dealloc_cmap'
     sh4-linux-ld: sh7760fb.c:(.text+0x814): undefined reference to `framebuffer_release'
  >> sh4-linux-ld: drivers/video/fbdev/sh7760fb.o:(.rodata+0xc): undefined reference to `fb_io_read'
  >> sh4-linux-ld: drivers/video/fbdev/sh7760fb.o:(.rodata+0x10): undefined reference to `fb_io_write'
     sh4-linux-ld: drivers/video/fbdev/sh7760fb.o:(.rodata+0x2c): undefined reference to `cfb_fillrect'
     sh4-linux-ld: drivers/video/fbdev/sh7760fb.o:(.rodata+0x30): undefined reference to `cfb_copyarea'
     sh4-linux-ld: drivers/video/fbdev/sh7760fb.o:(.rodata+0x34): undefined reference to `cfb_imageblit'

Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309130632.LS04CPWu-lkp@intel.com/
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230918090400.13264-1-tzimmermann@suse.de