gcc-plugins: Remove duplicate included header file stringpool.h

gcc-plugins: Remove duplicate included header file stringpool.h

The header file stringpool.h is included for GCC version >= 8 and then
again for all versions.

Since the header file stringpool.h was added in GCC 4.9 and the kernel
currently requires GCC 5.1 as a minimum, remove the conditional include.

Including the header file only once removes the following warning
reported by make includecheck:

  stringpool.h is included more than once

However, it's important to include stringpool.h before attribs.h
because attribs.h uses some of its functions.

Compile-tested with GCC 14.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Link: https://lore.kernel.org/r/20240629233608.278028-2-thorsten.blum@toblux.com
Signed-off-by: Kees Cook <kees@kernel.org>