sed_opal: Only build when we really have sed_opal headers

sed_opal: Only build when we really have sed_opal headers

There's already code to not build the sed opal plugin when the necessary
headers aren't present. However, it doesn't work, since HAVE_SED_OPAL is
always defined to be either 0 or 1, so the meson has() return true, even
when the value is 0, causing the sed-opal plugin to build, even when the
header files it needs are missing.

Signed-off-by: Warner Losh <imp@bsdimp.com>