mesa: Remove fprintf referring to libdxtn

When this file is included by Gallium, the fprintf causes it to fail to
compile. This is an unreachable error case, and we shouldn't be calling
fprintf directly.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/mesa/main/texcompress_s3tc_tmp.h

@@ -983,7 +983,7 @@ static void tx_compress_dxtn(GLint srccomps, GLint width, GLint height, const GL
       }
       break;
    default:
-      fprintf(stderr, "libdxtn: Bad dstFormat %d in tx_compress_dxtn\n", destFormat);
+      assert(false);
       return;
    }
 }