Need to declare zbuf as unsigned char * for mingw build

[elekven-m]

Due to changes in libplist's plist.h, plist_new_data now takes a first parameter which is unsigned char *, where it used to be signed char *. A build with gcc 13.2.0 in the mingw environment is defined to treat all warnings as errors, so the build breaks. Editing ideviceinstaller.c line 1211 to declare zbuf to be unsigned char * restores the build.