x86/boot: Add missing has_cpuflag() prototype

We get a warning when building the kernel with W=1:

  arch/x86/boot/compressed/cpuflags.c:4:6: warning: no previous prototype for ‘has_cpuflag’ [-Werror=missing-prototypes]
      4 | bool has_cpuflag(int flag)
        |      ^~~~~~~~~~~

Add a function declaration to cpuflags.h

Signed-off-by: Zhou Ding <zhouding@cmss.chinamobile.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241217162859.1167889-1-zhouding@cmss.chinamobile.com