builddeb: Skip gcc-plugins when not configured

When attempting to build a Debian kernel package, the "scripts/gcc-plugins"
directory does not exist in the output tree unless CONFIG_GCC_PLUGINS=y.
To avoid errors when not defined, this wraps the failing "find" in a config
test.

Reported-by: Frank Paulsen <frobnic+lkml@gmail.com>
Tested-by: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michal Marek <mmarek@suse.com>