configure: run plugin TCG tests again

Commit 39fb3cf ("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: 39fb3cf ("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 15cc103)
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")

configure

@@ -1675,6 +1675,9 @@ fi
 mkdir -p tests/tcg
 echo "# Automatically generated by configure - do not modify" > $config_host_mak
 echo "SRC_PATH=$source_path" >> $config_host_mak
+if test "$plugins" = "yes" ; then
+    echo "CONFIG_PLUGIN=y" >> $config_host_mak
+fi
 
 tcg_tests_targets=
 for target in $target_list; do