st/mesa: add MESA_SHADER_COMPUTE case in shader_stage_to_ptarget()

Silences compiler warning.  Trivial.

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -5027,6 +5027,8 @@ shader_stage_to_ptarget(gl_shader_stage stage)
       return PIPE_SHADER_FRAGMENT;
    case MESA_SHADER_GEOMETRY:
       return PIPE_SHADER_GEOMETRY;
+   case MESA_SHADER_COMPUTE:
+      return PIPE_SHADER_COMPUTE;
    }
 
    assert(!"should not be reached");