meson: fix megadriver symlinking

Which should be relative instead of absolute.

Fixes: f7f1b30f
       ("meson: extend install_megadrivers script to handle symmlinking")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105567


Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-and-Tested-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 6ac87c17)