gitweb: Set default destination directory for installing gitweb in Makefile

Currently installing gitweb requires to give a target directory (via 'gitwebdir' build variable). Giving it a default value protects against user errors. Signed-off-by: Pavan Kumar Sunkara <pavan.sss1991@gmail.com> Mentored-by: Christian Couder <chriscool@tuxfamily.org> Mentored-by: Petr Baudis <pasky@ucw.cz> Acked-by: Jakub Narebski <jnareb@gmail.com> Acked-by: Petr Baudis <pasky@suse.cz> Signed-off-by: Junio C Hamano <gitster@pobox.com>