st/mesa: advertise GL_ARB_half_float_pixel

This extension doesn't appear to need any driver-specific parts.

src/mesa/state_tracker/st_extensions.c

@@ -224,6 +224,7 @@ void st_init_extensions(struct st_context *st)
    ctx->Extensions.ARB_draw_elements_base_vertex = GL_TRUE;
    ctx->Extensions.ARB_fragment_coord_conventions = GL_TRUE;
    ctx->Extensions.ARB_fragment_program = GL_TRUE;
+   ctx->Extensions.ARB_half_float_pixel = GL_TRUE;
    ctx->Extensions.ARB_map_buffer_range = GL_TRUE;
    ctx->Extensions.ARB_multisample = GL_TRUE;
    ctx->Extensions.ARB_texture_border_clamp = GL_TRUE; /* XXX temp */