BUILD: thread: include log.h from thread.c

It's needed for ha_alert(). Probably that a separate file for error
reporting at boot would be useful.
diff --git a/src/thread.c b/src/thread.c
index fd9e6b6..bc655bd 100644
--- a/src/thread.c
+++ b/src/thread.c
@@ -26,6 +26,7 @@
 #include <haproxy/cfgparse.h>
 #include <haproxy/fd.h>
 #include <haproxy/global.h>
+#include <haproxy/log.h>
 #include <haproxy/thread.h>
 #include <haproxy/tools.h>