Moved the RetrieveSFMLClass function to the system module since several other modules will be using this function too.

git-svn-id: https://sfml.svn.sourceforge.net/svnroot/sfml/branches/sfml2@1678 4e206d99-4929-0410-ac5d-dfc041789085

bindings/ruby/sfml-system/system/main.hpp

@@ -25,6 +25,8 @@
 
 #include "ruby.h"
 
+VALUE RetrieveSFMLClass( const char * aName );
+
 // Ruby initiation function
 extern "C" void Init_system( void );