Simplify relative path in libGLU/Makefile

[Gerald Pfeifer]

I used some script that checks out ports individually where the
following patch simplified things, and in general it strikes me
as preferrable.

Okay to commit?

Gerald @FreeBSD.org

Index: libGLU/Makefile
===================================================================
RCS file: /home/pcvs/ports/graphics/libGLU/Makefile,v
retrieving revision 1.13
diff -r1.13 Makefile
26c26
< .include "${.CURDIR}/../../graphics/libGL/bsd.mesalib.mk"
---
> .include "${.CURDIR}/../libGL/bsd.mesalib.mk"