meson: Make sed optional

meson: Make sed optional

sed is used only if dtrace or selinux are enabled.  Those options are
only used on Unix platforms, which should have sed.  But we don't want
to make sed a hard requirement on Windows, which was the case in meson
until now.

This just changes sed to be not-required by meson.  If you happen to
use a system with, say, dtrace but without sed, you might get a
slightly complicated error from meson during the build, but that seems
better than making the requiredness a complicated conditional that
will need to be maintained.

Discussion: https://www.postgresql.org/message-id/flat/ZQzp_VMJcerM1Cs_%40paquier.xyz