syslinux: Disable error on implicit-function-declaration

syslinux: Disable error on implicit-function-declaration

syslinux has vendored copy of ext2fs/ext2_fs.h but uses ext2fs/ext2fs.h
from e2fsprogs package, however, ext2fs/ext2fs.h has dependencies on
ext2fs/ext2_fs.h coming from e2fsprogs package as these both headers
come from same package, here syslinux uses ext2fs.h from e2fsprogs but
supplies its own copy of ext2_fs.h which maybe out of sync and that
results in warnings about implicit implicit-function-declarations
e.g.

recipe-sysroot/usr/include/ext2fs/ext2fs.h:727:16: error: implicit declaration of function 'ext2fs_has_feature_gdt_csum' [-Wimplicit-function-declaration]
|   727 |                ext2fs_has_feature_gdt_csum(fs->super);
|       |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~

ext2fs_has_feature_gdt_csum here comes from newer version of
ext2fs/ext2_fs.h but missing from vendored copy, hence the warning.

With gcc-14 this warning is treated as error by default, which breaks
the build, so lets treat it as warning only.

All these functions are never used in syslinux, so functionality-wise we
are fine.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>