x86/sev: Guard sev_evict_cache() with CONFIG_AMD_MEM_ENCRYPT

The sev_evict_cache() is guest-related code and should be guarded by
CONFIG_AMD_MEM_ENCRYPT, not CONFIG_KVM_AMD_SEV.

CONFIG_AMD_MEM_ENCRYPT=y is required for a guest to run properly as an SEV-SNP
guest, but a guest kernel built with CONFIG_KVM_AMD_SEV=n would get the stub
function of sev_evict_cache() instead of the version that performs the actual
eviction. Move the function declarations under the appropriate #ifdef.

Fixes: 7b306dfa ("x86/sev: Evict cache lines during SNP memory validation")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@kernel.org # 6.16.x
Link: https://lore.kernel.org/r/70e38f2c4a549063de54052c9f64929705313526.1757708959.git.thomas.lendacky@amd.com