gitweb/Makefile: include in top-level Makefile

Include the gitweb/Makefile in the top-level Makefile rather than
calling it as a sub-Makefile. As noted in the thread starting at at
[1] (in particular [2]) we'll pay a high cost on NOOP runs of "make"
just to figure out that we have nothing to do for "make gitweb".

The "gitweb" script also isn't maintained out-of-tree, unlike
"gitk-git" or "git-gui", which both have their own "Makefile". Other
parts of it are already integrated into our main Makefiles, e.g. the
documentation is built by Documentation/Makefile since
07ea4df2780 (gitweb: Add gitweb(1) manpage for gitweb itself,
2011-10-16).

1. https://lore.kernel.org/git/20220525205651.825669-1-szeder.dev@gmail.com/
2. https://lore.kernel.org/git/220526.86k0a96sv2.gmgdl@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>