x86/hyperv: Add missing 'inline' to hv_snp_boot_ap() stub

When building without CONFIG_AMD_MEM_ENCRYPT, there are several
repeated instances of -Wunused-function due to missing 'inline' on
the stub of hy_snp_boot_ap():

  In file included from drivers/hv/hv_common.c:29:
  ./arch/x86/include/asm/mshyperv.h:272:12: error: 'hv_snp_boot_ap' defined but not used [-Werror=unused-function]
    272 | static int hv_snp_boot_ap(int cpu, unsigned long start_ip) { return 0; }
        |            ^~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Add 'inline' to fix the warnings.

Fixes: 44676bb9 ("x86/hyperv: Add smp support for SEV-SNP guest")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20230822-hv_snp_boot_ap-missing-inline-v1-1-e712dcb2da0f@kernel.org