glsl: add gl_LocalGroupSizeARB as a system value

v2: - only add it if the ext is enabled (Ilia)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/compiler/shader_enums.h

@@ -472,6 +472,7 @@ typedef enum
    SYSTEM_VALUE_GLOBAL_INVOCATION_ID,
    SYSTEM_VALUE_WORK_GROUP_ID,
    SYSTEM_VALUE_NUM_WORK_GROUPS,
+   SYSTEM_VALUE_LOCAL_GROUP_SIZE,
    /*@}*/
 
    /**