Add const qualifier to pathname member of TAR

The pathname member is only set onnce in tar init from a const char*, so
it can be const char* as well. This fixes the following compiler
warning:

handle.c: In function 'tar_init':
handle.c:47:17: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>