Initialize wxGtkValue in a way not requiring C++11

As we still support C++98, don't use G_VALUE_INIT as the initializer:
it's a braced-init-list, which is only allowed in C++11.

Co-Authored-By: Paul Cornett <paulcor@users.noreply.github.com>

include/wx/gtk/private/value.h

@@ -18,14 +18,14 @@ class wxGtkValue
 {
 public:
     explicit wxGtkValue()
-        : m_val(G_VALUE_INIT)
     {
+        memset(&m_val, 0, sizeof(m_val));
     }
 
     // Initialize the value of the specified type.
     explicit wxGtkValue(GType gtype)
-        : m_val(G_VALUE_INIT)
     {
+        memset(&m_val, 0, sizeof(m_val));
         g_value_init(&m_val, gtype);
     }