glsl: add the tessellation extension to the list for the "layout" qualifier

glsl: add the tessellation extension to the list for the "layout" qualifier

This is technically not needed, but it makes the compiler return a better
error message if tessellation is used with GLSL < 1.50.

Instead of:
    error: syntax error, unexpected NEW_IDENTIFIER, expecting $end
It returns:
    error: #version 150 layout qualifier `triangles' used

And the tessellation spec says:
    OpenGL 3.2 and GLSL 1.50 are required.
So it makes perfect sense.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>