makesyscalls.sh: always use absolute path for syscalls.conf

syscalls.conf is included using "." which per the Open Group:

If file does not contain a <slash>, the shell shall use the search
path specified by PATH to find the directory containing file.

POSIX shells don't fall back to the current working directory.

Submitted by: Nathaniel Wesley Filardo <nwf20@cl.cam.ac.uk>
Reviewed by: bdrewery
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D20476