Hide every symbol except ZNCModInfo from modules.

This means that now it's possible to have the same class names in
different modules, which reduces chances of weird errors.

Thanks to mclc for noticing this.

modules/modpython/module.h

@@ -15,6 +15,9 @@ public:
 
 class CModPython;
 
+#if HAVE_VISIBILITY
+#pragma GCC visibility push(default)
+#endif
 class CPyModule : public CModule {
 	PyObject* m_pyObj;
 	CModPython* m_pModPython;
@@ -224,3 +227,6 @@ public:
 	}
 };
 
+#if HAVE_VISIBILITY
+#pragma GCC visibility pop
+#endif