x86/cpu: Include the header of init_ia32_feat_ctl()'s prototype

Include the header containing the prototype of init_ia32_feat_ctl(),
solving the following warning:

  $ make W=1 arch/x86/kernel/cpu/feat_ctl.o
  arch/x86/kernel/cpu/feat_ctl.c:112:6: warning: no previous prototype for ‘init_ia32_feat_ctl’ [-Wmissing-prototypes]
    112 | void init_ia32_feat_ctl(struct cpuinfo_x86 *c)

This warning appeared after commit

  5d510359 ("x86/cpu: Reinitialize IA32_FEAT_CTL MSR on BSP during wakeup")

had moved the function init_ia32_feat_ctl()'s prototype from
arch/x86/kernel/cpu/cpu.h to arch/x86/include/asm/cpu.h.

Note that, before the commit mentioned above, the header include "cpu.h"
(arch/x86/kernel/cpu/cpu.h) was added by commit

  0e79ad86 ("x86/cpu: Fix a -Wmissing-prototypes warning for init_ia32_feat_ctl()")

solely to fix init_ia32_feat_ctl()'s missing prototype. So, the header
include "cpu.h" is no longer necessary.

  [ bp: Massage commit message. ]

Fixes: 5d510359

 ("x86/cpu: Reinitialize IA32_FEAT_CTL MSR on BSP during wakeup")
Signed-off-by: Luciano Leão <lucianorsleao@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nícolas F. R. A. Prado <n@nfraprado.net>
Link: https://lore.kernel.org/r/20220922200053.1357470-1-lucianorsleao@gmail.com