meson: use better zlib dependency fallback

meson: use better zlib dependency fallback

zlib is required, and if it isn't found it is checked several ways and
then forced via subproject(). This code was added in commit
b93e37592a3ccc0eaece1c8fef2d362b1e5fe685, to account for systems where
zlib doesn't have pkg-config files installed.

But Meson already does dependency fallback, and also, since 0.54.0, does
the in-between checks for find_library('z') and has_header('zlib.h') via
the "system" type dependency. Simplify dependency lookup by marking it
as required, which also makes sure that the console log doesn't
confusingly list "not found".

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2484>