Fix compiler warnings.

In our build environment, some UN*Xes define the historic index()
routine in addition to the standard strchr() routine.

Change-Id: Id39ad42a60def6af4c5b3625528207e9d2948392
Reviewed-on: https://code.wireshark.org/review/4002
Reviewed-by: Guy Harris <guy@alum.mit.edu>