InitializeOnceHelper: use std::function instead of C function pointer

InitializeOnceHelper calls Loader::AddDeferredInitializer which takes a
std::function, so it's eventually converted to that anyways. This commit just
does this a bit earlier, and by saving the step of the intermediate C function
pointer, this would now also work for capturing lambdas (which there are none
of at the moment).

lib/base/initialize.cpp

@@ -5,7 +5,7 @@
 
 using namespace icinga;
 
-bool icinga::InitializeOnceHelper(void (*func)(), int priority)
+bool icinga::InitializeOnceHelper(const std::function<void()>& func, int priority)
 {
 	Loader::AddDeferredInitializer(func, priority);
 	return true;

lib/base/initialize.hpp

@@ -4,6 +4,7 @@
 #define INITIALIZE_H
 
 #include "base/i2-base.hpp"
+#include <functional>
 
 namespace icinga
 {
@@ -13,7 +14,7 @@ namespace icinga
 
 #define I2_UNIQUE_NAME(prefix) I2_TOKENPASTE2(prefix, __COUNTER__)
 
-bool InitializeOnceHelper(void (*func)(), int priority = 0);
+bool InitializeOnceHelper(const std::function<void()>& func, int priority = 0);
 
 #define INITIALIZE_ONCE(func)									\
 	namespace { namespace I2_UNIQUE_NAME(io) {							\