thread/Thread: make IsInside() debug-only

This method is only used inside assert().

src/IOThread.hxx

@@ -20,6 +20,7 @@
 #ifndef MPD_IO_THREAD_HXX
 #define MPD_IO_THREAD_HXX
 
+#include "check.h"
 #include "Compiler.h"
 
 class EventLoop;
@@ -53,6 +54,8 @@ gcc_const
 EventLoop &
 io_thread_get() noexcept;
 
+#ifndef NDEBUG
+
 /**
  * Is the current thread the I/O thread?
  */
@@ -61,3 +64,5 @@ bool
 io_thread_inside() noexcept;
 
 #endif
+
+#endif