Fix the virtual table tests compilation under GCC.

Technically we should also be defining the copy constructors but they're not used by the bindings.

Error caught by Travis:

1) Test Error : VTablesTests.TestVirtualFuntionRetVal
   System.EntryPointNotFoundException : _ZN19DerivedClassVirtualC2Ev

tests/VTables/VTables.cpp

@@ -34,6 +34,10 @@ int FooCallFoo(Foo* foo)
     return foo->vfoo() + 2;
 }
 
+BaseClassVirtual::BaseClassVirtual()
+{
+}
+
 int BaseClassVirtual::virtualCallRetInt(BaseClassVirtual* base)
 {
     return base->retInt();
@@ -54,6 +58,10 @@ BaseClassVirtual* BaseClassVirtual::getBasePtr()
     return new DerivedClassVirtual();
 }
 
+DerivedClassVirtual::DerivedClassVirtual()
+{
+}
+
 int DerivedClassVirtual::retInt()
 {
     return 10;

tests/VTables/VTables.h

@@ -22,6 +22,7 @@ DLL_API int FooCallFoo(Foo* foo);
 class DLL_API BaseClassVirtual
 {
 public:
+    BaseClassVirtual();
     static int virtualCallRetInt(BaseClassVirtual* base);
     virtual int retInt();
     static BaseClassVirtual getBase();
@@ -31,5 +32,6 @@ public:
 class DLL_API DerivedClassVirtual : public BaseClassVirtual
 {
 public:
+    DerivedClassVirtual();
     virtual int retInt() override;
 };