Prevent recursion when signaling EVENT_LOG event

If a plugin hooked on EVENT_LOG calls log_event() directly or indirectly
and the log level settings include the level for the event being logged,
MantisBT enters in an endless recursion loop.

Using a static variable to prevent recursion if EVENT_LOG has already
been signaled prevents this.

Fixes 0032243
Fixes https://github.com/mantisbt-plugins/EventLog/issues/3