[exim] index() function use in 4.83

I belatedly updated to 4.83 last Tuesday, and encountered one minor issue
which is not fixed in 4.84RC.

The malware.c file contains two references to the "index" function, an
obsolete alias for strchr. As I run a stripped down uClibc with no
index(), this blocked compilation, although it was easy to patch.

These should be changed to strchr() in 4.84.

---- Michael Deutschmann <michael@???>