Belt printer: declare up_direction in both phong fragment shaders (#16226)

Fixes the `unable to load shaders: phong` error at startup on
`belt-printer` after #16195. That PR added `vec3 up_direction` to the
`SlopeDetection` uniform struct in `phong.vs` (110 and 140) but not in
`phong.fs`, so the vertex and fragment stages declared the `slope`
uniform with different struct types and the program failed to link.
`gouraud.fs` already carried the member; `phong.fs` now does too.
Shader-only change.

resources/shaders/110/phong.fs

@@ -41,6 +41,7 @@ struct SlopeDetection
     bool actived;
     float normal_z;
     mat3 volume_world_normal_matrix;
+    vec3 up_direction;
 };
 
 uniform vec4 uniform_color;

resources/shaders/140/phong.fs

@@ -44,6 +44,7 @@ struct SlopeDetection
     bool actived;
     float normal_z;
     mat3 volume_world_normal_matrix;
+    vec3 up_direction;
 };
 
 uniform vec4 uniform_color;