mail/bincimap: unbreak with libc++ 3.9

In file included from address.cc:39:
./convert.h:117:14: error: assigning to 'char *' from incompatible type 'const char *'

if ((t = strchr(hexchars, c)) == 0)
       ^ ~~~~~~~~~~~~~~~~~~~

./convert.h:122:14: error: assigning to 'char *' from incompatible type 'const char *'

if ((t = strchr(hexchars, d)) == 0)
       ^ ~~~~~~~~~~~~~~~~~~~

Reported by: pkg-fallout