fix Expressions assignment for FreeBSD

Just like Apple, FreeBSD uses clang as it's main compiler, and thus
runs afoul of the same problem in src/App/PropertyExpressionEngine.cpp
like Apple (issue #5281): "error: no viable overloaded '='".
Extend the fix from #5281 for the BSDs (check macro FC_OS_BSD) -
at least OpenBSD uses clang, too (maybe this check should be reworked
to test for clang, but I can't test on Apple, so let's keep this fix
minimalistic).

References: #5281

src/App/PropertyExpressionEngine.h

@@ -179,7 +179,7 @@ private:
     typedef boost::adjacency_list< boost::listS, boost::vecS, boost::directedS > DiGraph;
     typedef std::pair<int, int> Edge;
     // Note: use std::map instead of unordered_map to keep the binding order stable
-    #ifdef FC_OS_MACOSX
+    #if defined(FC_OS_MACOSX) || defined(FC_OS_BSD)
     typedef std::map<App::ObjectIdentifier, ExpressionInfo> ExpressionMap;
     #else
     typedef std::map<const App::ObjectIdentifier, ExpressionInfo> ExpressionMap;