Newest GCC/glibc requires fixes for hplip-3.9.4b

[Johannes Meixner]

With glibc-2.10
prnt/hpijs/dj3320.cpp and prnt/hpijs/registry.cpp result
"invalid conversion from 'const char*' to 'char*'" errors during build.

For background information see
http://udrepper.livejournal.com/20948.html

Basically
char *strchr(char *, int)
must be replaced with
const char *strchr(const char *, int)
when the first argument is of type const char.

[Johannes Meixner (jsmeix)]

The attached patch lets it build for me without errors.