Make sure has_encrypted_entries is a signed int

Related to cd1740912b18f748a4f9b491a32b44c1712a7478

Trying to assign -1 (ARCHIVE_READ_FORMAT_ENCRYPTION_DONT_KNOW)
to a data type we're not sure will always be signed is a bad idea
(blew up on ARM, for instance).
Merge pull request #57 from NiLuJe/fix-has_encrypted_entries

Make sure has_encrypted_entries is a signed int