x86/boot: Don't disable PV32 when XEN_SHSTK is compiled out

There is no need to automatically disable PV32 support on SHSTK-capable
hardware if Xen isn't actually using the feature.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Paul Durrant <paul@xen.org>

xen/arch/x86/setup.c

@@ -95,7 +95,11 @@ unsigned long __initdata highmem_start;
 size_param("highmem-start", highmem_start);
 #endif
 
+#ifdef CONFIG_XEN_SHSTK
 static bool __initdata opt_xen_shstk = true;
+#else
+#define opt_xen_shstk false
+#endif
 
 static int __init parse_cet(const char *s)
 {