x86: Avoid missing-prototype warnings for doublefault code

Two functions in the 32-bit doublefault code are lacking a prototype:

arch/x86/kernel/doublefault_32.c:23:36: error: no previous prototype for 'doublefault_shim' [-Werror=missing-prototypes]
   23 | asmlinkage noinstr void __noreturn doublefault_shim(void)
      |                                    ^~~~~~~~~~~~~~~~
arch/x86/kernel/doublefault_32.c:114:6: error: no previous prototype for 'doublefault_init_cpu_tss' [-Werror=missing-prototypes]
  114 | void doublefault_init_cpu_tss(void)

The first one is only called from assembler, while the second one is
declared in doublefault.h, but this file is not included.

Include the header file and add the other declaration there as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/all/20230516193549.544673-8-arnd%40kernel.org