link seccomp only with softmmu targets

Now, if seccomp is detected, it is linked into every executable,
but is used only by softmmu targets (from vl.c).  So link it
only where it is actually needed.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

configure

@@ -1434,7 +1434,7 @@ fi
 
 if test "$seccomp" != "no" ; then
     if $pkg_config --atleast-version=1.0.0 libseccomp --modversion >/dev/null 2>&1; then
-        LIBS=`$pkg_config --libs libseccomp`
+        libs_softmmu="$libs_softmmu `$pkg_config --libs libseccomp`"
 	seccomp="yes"
     else
 	if test "$seccomp" = "yes"; then