meson: fix zstd dependency

We have a meson option (http-zstd) to enable or disable support for zstd
compression, but this was not being applied properly.  If libzstd was
found by meson, the feature would still be enabled even if it was
`-Dhttp-zstd=disabled`.  Fix the handling of the feature flag so that
if the feature is disabled it is explicitly not searched for as a
dependency.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8260bdb7d160c04f9ac2b8fce5ffb2abbf1c3c1a