Makefile: Fix 'clean' target to remove all gitweb build files

In particular the gitweb/GITWEB-BUILD-OPTIONS file was not being removed by the main Makefile. However, the gitweb/Makefile has a 'clean' target that correctly removes all the build products. In order to fix the problem, rather than duplicate the clean-up instructions, we change the main Makefile so that it delegates the clean-up actions to the gitweb Makefile. Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk> Acked-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>