misc: Fix nesting of namespace{} and if-endif.

Fix a build error caused by the misaligned nesting of namespace{} and
if-endif. Previously, if HAVE_TUNTAP is not defined then the gem5
namespace is not closed.

Change-Id: I5e45d2271f97bb9e38c91565adb6aff0a7b43744
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47761
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/net/ethertap.cc

@@ -472,6 +472,6 @@ EtherTap::sendReal(const void *data, size_t len)
     return true;
 }
 
-} // namespace gem5
+#endif // HAVE_TUNTAP
 
-#endif
+} // namespace gem5