meson: stop linking libcurl into all executables

meson: stop linking libcurl into all executables

We set up libcurl via the `libgit_dependencies` variable, which gets
propagated into every user of the `libgit` dependency. This is not
necessary though, as most of our executables aren't even supposed to
link against libcurl.

Fix this by only propagating include directories as a libgit dependency
and propagating the full curl dependency via `libgit_curl`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>