Provide std::default_delete<GWBUF*>

Now GWBUFs can be placed in std::unique_ptr:s.

include/maxscale/buffer.hh

@@ -15,11 +15,26 @@
 #include <maxscale/ccdefs.hh>
 #include <algorithm>
 #include <iterator>
+#include <memory>
 #include <new>
 #include <vector>
 #include <maxbase/assert.h>
 #include <maxscale/buffer.h>
 
+namespace std
+{
+
+template<>
+struct default_delete<GWBUF>
+{
+    void operator()(GWBUF* pBuffer)
+    {
+        gwbuf_free(pBuffer);
+    }
+};
+
+}
+
 namespace maxscale
 {