Compiler Attributes: counted_by: bump min gcc version

GCC is expected to implement this feature in version 15,
so bump the version.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/e1c27b64ae7abe2ebe647be11b71cf1bca84f677.1704855495.git.senozhatsky@chromium.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

include/linux/compiler_attributes.h

@@ -95,7 +95,7 @@
 #endif
 
 /*
- * Optional: only supported since gcc >= 14
+ * Optional: only supported since gcc >= 15
  * Optional: only supported since clang >= 18
  *
  *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896