Compiler Attributes: add missing SPDX ID in compiler_types.h

Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # on top of v4.19-rc5, clang 7
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: NLuc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: NMiguel Ojeda <miguel.ojeda.sandonis@gmail.com>

include/linux/compiler_types.h

+/* SPDX-License-Identifier: GPL-2.0 */
 #ifndef __LINUX_COMPILER_TYPES_H
 #define __LINUX_COMPILER_TYPES_H