auxdisplay: charlcd: Don't rebuild when CONFIG_PANEL_BOOT_MESSAGE=y

When CONFIG_PANEL_BOOT_MESSAGE=y the module still includes
the generated header and gets rebuilt even if it doesn't use
anything from that header.  Include generated header conditionally
to avoid unnecessary rebuilds.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>