mesa/main: Fix missing return in non void function

This was found by obs:
I: Program returns random data in a function
E: Mesa no-return-in-nonvoid-function main/program_resource.c:109

v2: Remove the ! on the string (Ian Romanick)

Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/program_resource.c

@@ -104,7 +104,7 @@ stage_from_program_interface(GLenum programInterface)
    case GL_COMPUTE_SUBROUTINE_UNIFORM:
       return MESA_SHADER_COMPUTE;
    default:
-      assert(!"unexpected programInterface value");
+      unreachable("unexpected programInterface value");
    }
 }