model: Add use of "extern template" to prevent duplicate instantiation/initialization of templated classes

This syntax has been tested with gcc and clang, but may not be fully
supported by other compilers.  Additional testing (and optional ifdef
blocks) is required for the final solution to the problem.
diff --git a/model/fw/per-fib-limits.cc b/model/fw/per-fib-limits.cc
index 3ec4f57..f2fed86 100644
--- a/model/fw/per-fib-limits.cc
+++ b/model/fw/per-fib-limits.cc
@@ -33,6 +33,10 @@
 namespace ndn {
 namespace fw {
 
+extern template class PerOutFaceLimits<BestRoute>;
+extern template class PerOutFaceLimits<Flooding>;
+extern template class PerOutFaceLimits<SmartFlooding>;
+
 template class PerFibLimits< PerOutFaceLimits<BestRoute> >;
 typedef PerFibLimits< PerOutFaceLimits<BestRoute> > PerFibLimitsPerOutFaceLimitsBestRoute;
 NS_OBJECT_ENSURE_REGISTERED (PerFibLimitsPerOutFaceLimitsBestRoute);