graphics/glew: restore GLU for now after 8532b6f5d078

While glew itself builds with -DGLEW_NO_GLU it's neither passed via
pkg-config to consumers nor visible outside of pkg-config.

$ pkg install glew pkgconf
$ pkg-config --cflags glew
Package glu was not found in the pkg-config search path.
Perhaps you should add the directory containing `glu.pc'
to the PKG_CONFIG_PATH environment variable
Package 'glu', required by 'glew', not found

PR: 257626 (for tracking)
Reported by: madpilot