ofono: fix the build when toolchain has old linux headers

ofono: fix the build when toolchain has old linux headers

Whilst our default toolchain has modern kernel headers (6.12, at time of
writing), some external toolchains may use old kernel headers.

As ofono's rmnet module uses kernel defines which were added in 5.14,
add some compatibility defines in case they are not set.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>