Only expose clean_backtrace() if we'll implement it

Fixes windows compilation; bug not in any released Tor.

Bugfix on cc9e86db.

src/common/backtrace.h

@@ -11,8 +11,11 @@ int configure_backtrace_handler(const char *tor_version);
 void clean_up_backtrace_handler(void);
 
 #ifdef EXPOSE_CLEAN_BACKTRACE
+#if defined(HAVE_EXECINFO_H) && defined(HAVE_BACKTRACE) && \
+  defined(HAVE_BACKTRACE_SYMBOLS_FD) && defined(HAVE_SIGACTION)
 void clean_backtrace(void **stack, int depth, const ucontext_t *ctx);
 #endif
+#endif
 
 #endif