irc/hexchat: Some fixes to port

- Use upstream release archive as recommended by Porters Handbook
  (USE_GITHUB section)
- Define gdkpixbuf2 dependency (reported by Poudriere)
- Adjust SHEBANG_FILES to remove the need to define BINARY_ALIAS
- Remove INSTALL_TARGET
- Define INSTALLS_ICONS (reported by Portlint)
- Remove unnecessary LIBS variable

PR:		261922
Reviewed by:	pkubaj (maintainer)