Declared state_base::~state_base virtual for GCC. This makes state exit slightly slower but does away with loads of "has virtual functions but non-virtual destructor" warnings.

[SVN r34047]

libs/statechart

@@ -1 +1 @@
-Subproject commit 798293b09707646de63ccf428a97d730b167e098
+Subproject commit bb0c4ad604bb5dd7ae9102c524dee3729f7c6dc3