x86, xen: Initialize cx to suppress warning

Initialize cx before calling xen_cpuid(), in order to suppress the
"may be used uninitialized in this function" warning.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>

arch/x86/xen/enlighten.c

@@ -215,6 +215,7 @@ static __init void xen_init_cpuid_mask(void)
 			  (1 << X86_FEATURE_ACPI));  /* disable ACPI */
 
 	ax = 1;
+	cx = 0;
 	xen_cpuid(&ax, &bx, &cx, &dx);
 
 	/* cpuid claims we support xsave; try enabling it to see what happens */