glsl: Add ARB_shader_image_load_store extension enables.

Reviewed-by: Paul Berry <stereotype441@gmail.com>

src/glsl/glsl_parser_extras.cpp

    EXT(AMD_shader_trinary_minmax,      true,  false,     dummy_true),
    EXT(ARB_viewport_array,             true,  false,     ARB_viewport_array),
    EXT(ARB_compute_shader,             true,  false,     ARB_compute_shader),
+   EXT(ARB_shader_image_load_store,    true,  false,     ARB_shader_image_load_store),
 };
 
 #undef EXT

src/glsl/glsl_parser_extras.h

    bool ARB_viewport_array_warn;
    bool ARB_compute_shader_enable;
    bool ARB_compute_shader_warn;
+   bool ARB_shader_image_load_store_enable;
+   bool ARB_shader_image_load_store_warn;
    /*@}*/
 
    /** Extensions supported by the OpenGL implementation. */