ControlManager: Fix a class/struct redefinition warning

Found by Clang 14 on FreeBSD CI:

kmix/core/ControlManager.cpp:26:1: warning: 'Listener' defined as
a struct here but previously declared as a class; this is valid,
but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]
struct Listener
^
kmix/core/ControlManager.h:29:1: note: did you mean struct here?
class Listener;
^