configure: fix seccomp check

Currently, if libseccomp is missing but the user explicitly requested
seccomp support using --enable-seccomp, configure silently ignores the
situation and disables seccomp support.

This is unlike all other tests that explicitly fail in such situation.

Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>

configure

@@ -1431,10 +1431,10 @@ if test "$seccomp" != "no" ; then
         LIBS=`$pkg_config --libs libseccomp`
 	seccomp="yes"
     else
-	seccomp="no"
 	if test "$seccomp" = "yes"; then
             feature_not_found "libseccomp"
 	fi
+	seccomp="no"
     fi
 fi
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