Make the port build with the upcoming Qt 4.8.x series.

Since 4.8.0 [1], Qt itself does not automatically include GLU's
headers and libraries with its OpenGL module, so applications which
relied on Qt to get these need to be adjusted.

Contrary to other ports, PORTREVISION was not bumped here because
freemat already depended on glu, we just needed to add a missing
#include.

[1] http://qt.gitorious.org/qt/qt/commit/e7eed096a0c33607a7a37baaf06e5952dc9d556b

Approved by: miwi (needed for the Qt 4.8.1 exp-run)