x86/boot/string: Add missing function prototypes

Silence "warning: no previous prototype for ... [-Wmissing-prototypes]"
warnings from string.h when building under W=1.

 [ bp: Clarify commit message. ]

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211119175325.3668419-1-keescook@chromium.org