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

Add a missing include in order to fix -Wmissing-prototypes warning:

  arch/x86/kernel/cpu/feat_ctl.c:95:6: warning: no previous prototype for ‘init_ia32_feat_ctl’ [-Wmissing-prototypes]
     95 | void init_ia32_feat_ctl(struct cpuinfo_x86 *c)

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200323105934.26597-1-b.thiel@posteo.de