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>

Author: toblux

scripts/gcc-plugins/gcc-common.h

@@ -62,11 +62,7 @@
 #include "pass_manager.h"
 #include "predict.h"
 #include "ipa-utils.h"
-
-#if BUILDING_GCC_VERSION >= 8000
 #include "stringpool.h"
-#endif
-
 #include "attribs.h"
 #include "varasm.h"
 #include "stor-layout.h"
@@ -78,7 +74,6 @@
 #include "context.h"
 #include "tree-ssa-alias.h"
 #include "tree-ssa.h"
-#include "stringpool.h"
 #if BUILDING_GCC_VERSION >= 7000
 #include "tree-vrp.h"
 #endif