rtnetlink: Remove executable bit from rtnetlink.h

In commit 135ee7e ("rtnetlink: extend parser to include kind of master and
slave") the file mode of rtnetlink.h accidentaly changed from 0644 to 0755.

This commit restores the previous file mode (0644) on rtnetlink.h.

CC: John Hurley <john.hurley@netronome.com>
Fixes: 135ee7e ("rtnetlink: extend parser to include kind of master and slave")
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>