add comment to r23998 code change to be able to track libevent code change better

This commit was SVN r24005.

The following SVN revision numbers were found above:
  r23998 --> open-mpi/ompi@e8aa8984a8

opal/mca/event/libevent207/libevent/include/event2/event.h

@@ -45,6 +45,9 @@ extern "C" {
 #include <sys/time.h>
 #endif
 #ifndef WIN32
+/** OMPI 
+ * wrap include stdbool.h with ifdef to prevent certain C++ compilers choking on bool redefine 
+ */
 #    if !(defined(c_plusplus) || defined(__cplusplus))
 #        include <stdbool.h>
 #    endif