tty: vt: conmakehash: Don't mention the full path of the input in output

tty: vt: conmakehash: Don't mention the full path of the input in output

This change strips $abs_srctree of the input file containing the
character mapping table in the generated output. The motivation for this
change is Yocto emitting a build warning

        WARNING: linux-lxatac-6.7-r0 do_package_qa: QA Issue: File /usr/src/debug/linux-lxatac/6.7-r0/drivers/tty/vt/consolemap_deftbl.c in package linux-lxatac-src contains reference to TMPDIR

So this change brings us one step closer to make the build result
reproducible independent of the build path.

Signed-off-by: Uwe Kleine-König <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>