graphics/zphoto: unbreak with libc++ 3.9

image.cpp:557:11: error: cannot initialize a variable of type 'char *' with an rvalue of type 'const char *'

char *suffix1 = strrchr(src,  '.');
      ^         ~~~~~~~~~~~~~~~~~~

image.cpp:558:11: error: cannot initialize a variable of type 'char *' with an rvalue of type 'const char *'

char *suffix2 = strrchr(dest, '.');
      ^         ~~~~~~~~~~~~~~~~~~

PR: 212343
Reported by: antoine (via exp-run)