gcc-plugins: Add include required by GCC release 8

commit 80d172431696482d9acd8d2c4ea78fed8956e2a1 upstream.

GCC requires another #include to get the gcc-plugins to build cleanly.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Lance Albertson <lance@osuosl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>