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>

src/glsl/glsl_lexer.ll

@@ -426,7 +426,8 @@ layout		{
 		      || yyextra->ARB_uniform_buffer_object_enable
 		      || yyextra->ARB_fragment_coord_conventions_enable
                       || yyextra->ARB_shading_language_420pack_enable
-                      || yyextra->ARB_compute_shader_enable) {
+                      || yyextra->ARB_compute_shader_enable
+                      || yyextra->ARB_tessellation_shader_enable) {
 		      return LAYOUT_TOK;
 		   } else {
 		      void *mem_ctx = yyextra;