git-instaweb: Simplify build dependency on gitweb

Since c0cb4ed (git-instaweb: Configure it to work with new gitweb
structure, 2010-05-28) git-instaweb does not re-create gitweb.cgi
etc., but makes use of installed gitweb. Therefore simplify
git-instaweb dependency on gitweb subsystem in main Makefile from
'gitweb/gitweb.cgi gitweb/static/gitweb.css gitweb/static/gitweb.js'
to simply 'gitweb'.

This is preparation for splitting gitweb.perl script, and for
splitting gitweb.js (to be reassembled / combined on build). This way
we don't have to duplicate parts of gitweb/Makefile in main
Makefile... it is also more correct description of git-instaweb
dependency.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>