Commit 39fb3cfc28b ("configure: clean up plugin option handling", 2023-10-18)
dropped the CONFIG_PLUGIN line from tests/tcg/config-host.mak, due to confusion
caused by the shadowing of $config_host_mak.  However, TCG tests were still
expecting it.  Oops.

Put it back, in the meanwhile the shadowing is gone so it's clear that it goes
in the tests/tcg configuration.

Cc:  <alex.bennee@linaro.org>
Fixes: 39fb3cfc28b ("configure: clean up plugin option handling", 2023-10-18)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20240124115332.612162-1-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240207163812.3231697-4-alex.bennee@linaro.org>
(cherry picked from commit 15cc103362499bd94c5aec5fa66543d0de3bf4b5)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: fixup for 8.2.x - $config_host_mak without tests/tcg/ prefix -
 for before v8.2.0-142-g606c3ba7a2 "configure: remove unnecessary subshell")