configure: usbredir fixes

usbredir is only used by system emulation, so add the libraries to
libs_softmmu instead of LIBS.

Cc: Michael Tokarev <mjt@tls.msk.ru>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

configure

@@ -2763,7 +2763,7 @@ if test "$usb_redir" != "no" ; then
         usb_redir_cflags=$($pkg_config --cflags libusbredirparser 2>/dev/null)
         usb_redir_libs=$($pkg_config --libs libusbredirparser 2>/dev/null)
         QEMU_CFLAGS="$QEMU_CFLAGS $usb_redir_cflags"
-        LIBS="$LIBS $usb_redir_libs"
+        libs_softmmu="$libs_softmmu $usb_redir_libs"
     else
         if test "$usb_redir" = "yes"; then
             feature_not_found "usb-redir"