glsl: Fix gl_shader_stage enum unsigned comparison

Replace -1 with MESA_SHADER_NONE enum value to fix sign related warning:

external/mesa3d/src/compiler/glsl/link_varyings.cpp:1415:25: warning: comparison of constant -1 with expression of type 'gl_shader_stage' is always true [-Wtautological-constant-out-of-range-compare]
        (consumer_stage != -1 && consumer_stage != MESA_SHADER_FRAGMENT))) {
         ~~~~~~~~~~~~~~ ^  ~~

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Rob Herring <robh@kernel.org>

src/compiler/shader_enums.h

@@ -39,6 +39,7 @@ extern "C" {
  */
 typedef enum
 {
+   MESA_SHADER_NONE = -1,
    MESA_SHADER_VERTEX = 0,
    MESA_SHADER_TESS_CTRL = 1,
    MESA_SHADER_TESS_EVAL = 2,