video: fbdev: controlfb: Fix build for COMPILE_TEST=y && PPC_PMAC=n

The build is currently broken, if COMPILE_TEST=y and PPC_PMAC=n:

  linux/drivers/video/fbdev/controlfb.c: In function ‘control_set_hardware’:
  linux/drivers/video/fbdev/controlfb.c:276:2: error: implicit declaration of function ‘btext_update_display’
    276 |  btext_update_display(p->frame_buffer_phys + CTRLFB_OFF,
        |  ^~~~~~~~~~~~~~~~~~~~

Fix it by including btext.h whenever CONFIG_BOOTX_TEXT is enabled.

Fixes: a07a63b0 ("video: fbdev: controlfb: add COMPILE_TEST support")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://lore.kernel.org/r/20200821104910.3363818-1-mpe@ellerman.id.au