Add missing const to CAddrMan::Check_()

Also: Always compile the function signature to avoid similar issues in
the future.

src/addrman.h

@@ -738,19 +738,15 @@ private:
     void Check() const
         EXCLUSIVE_LOCKS_REQUIRED(cs)
     {
-#ifdef DEBUG_ADDRMAN
         AssertLockHeld(cs);
         const int err = Check_();
         if (err) {
             LogPrintf("ADDRMAN CONSISTENCY CHECK FAILED!!! err=%i\n", err);
         }
-#endif
     }
 
-#ifdef DEBUG_ADDRMAN
     //! Perform consistency check. Returns an error code or zero.
     int Check_() const EXCLUSIVE_LOCKS_REQUIRED(cs);
-#endif
 
     /**
      * Return all or many randomly selected addresses, optionally by network.