package/mesa3d/mesa3d.mk: fix typo in comment

Commit 6a51177a (package/mesa3d: always enable glx-direct if glx is
enabled) introduced a typo in the comment, fix that.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>