notmuch-config: use strchr(), not index() (Solaris support)
notmuch-config.c has the only use of the function named "index()" in the
notmuch source. Several other places use the equivalent function
"strchr()"; this patch just fixes notmuch-config.c to use strchr()
instead. (Solaris needs to include <strings.h> to get the prototype for
index(), and notmuch-config.c was failing to include that header, so it
wasn't compiling as-is.)