KVM: SVM: Fix build error when using -Werror=unused-but-set-variable

Commit 916e3e5f ("KVM: SVM: Do not use user return MSR support for
virtualized TSC_AUX") introduced a local variable used for the rdmsr()
function for the high 32-bits of the MSR value. This variable is not used
after being set and triggers a warning or error, when treating warnings
as errors, when the unused-but-set-variable flag is set. Mark this
variable as __maybe_unused to fix this.

Fixes: 916e3e5f ("KVM: SVM: Do not use user return MSR support for virtualized TSC_AUX")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <0da9874b6e9fcbaaa5edeb345d7e2a7c859fc818.1696271334.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/svm.c

@@ -691,7 +691,7 @@ static int svm_hardware_enable(void)
 	 */
 	if (boot_cpu_has(X86_FEATURE_V_TSC_AUX)) {
 		struct sev_es_save_area *hostsa;
-		u32 msr_hi;
+		u32 __maybe_unused msr_hi;
 
 		hostsa = (struct sev_es_save_area *)(page_address(sd->save_area) + 0x400);