scripts: dtc: libfdt: fdt_ro.c: always define fdt_check_full

On some configs (like stm32mp15_dhcom_basic_defconfig), if configs
SPL_LOAD_FIT_FULL and SPL_FIT_FULL_CHECK are enabled. Then the compilatio
fails with the following error:

arm-linux-gnueabi-ld.bfd: boot/image-fit.o: in function `fit_check_format':
<PATH>/uboot/u-boot-stm/boot/image-fit.c:1641: undefined reference to `fdt_check_full'
scripts/Makefile.spl:509: recipe for target 'spl/u-boot-spl' failed

This issue happens because the function fdt_check_full is only defined if
"!defined(FDT_ASSUME_MASK) || FDT_ASSUME_MASK != 0xff". But this function
may be called even if this condition are not verified. To avoid this issue,
the function fdt_check_full is always defined.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>