BUILD: thread: implement thread_harmless_end_sig() for threadless builds

Building without thread support was broken in 2.8-dev2 with commit
7e70bfc8c ("MINOR: threads: add a thread_harmless_end() version that
doesn't wait") that forgot to define the function for the threadless
cases. No backport is needed.
diff --git a/include/haproxy/thread.h b/include/haproxy/thread.h
index 6590706..67ceba3 100644
--- a/include/haproxy/thread.h
+++ b/include/haproxy/thread.h
@@ -125,6 +125,10 @@
 {
 }
 
+static inline void thread_harmless_end_sig()
+{
+}
+
 static inline void thread_isolate()
 {
 }