mesa: Make gl_VertexID be a system value like gl_InstanceID.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
diff --git a/src/glsl/builtin_variables.cpp b/src/glsl/builtin_variables.cpp
index 0d804c2..ed6b922 100644
--- a/src/glsl/builtin_variables.cpp
+++ b/src/glsl/builtin_variables.cpp
@@ -98,7 +98,7 @@
 };
 
 static const builtin_variable builtin_130_vs_variables[] = {
-   { ir_var_in,  -1,                 "int",   "gl_VertexID" },
+   { ir_var_system_value,  SYSTEM_VALUE_VERTEX_ID, "int",   "gl_VertexID" },
 };
 
 static const builtin_variable builtin_110_deprecated_uniforms[] = {