base: explicitly suggest potential use of 'All' debug flags

Without this declaration, new clangs will complain about this value
being unused. It has no explicit use in the codebase, but it can be
useful to turn on all debugging flags while in a debugger to greatly
increase simulator verbosity.

src/base/debug.hh

@@ -116,6 +116,8 @@ FlagsMap &allFlags();
 
 Flag *findFlag(const std::string &name);
 
+extern Flag *const All;
+
 bool changeFlag(const char *s, bool value);
 
 } // namespace Debug