x86/asm/32: Fix ANNOTATE_UNRET_SAFE use on 32-bit

The build on x86_32 currently fails after commit

  9bb2ec60 (objtool: Update Retpoline validation)

with:

  arch/x86/kernel/../../x86/xen/xen-head.S:35: Error: no such instruction: `annotate_unret_safe'

ANNOTATE_UNRET_SAFE is defined in nospec-branch.h. And head_32.S is
missing this include. Fix this.

Fixes: 9bb2ec60 ("objtool: Update Retpoline validation")
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/63e23f80-033f-f64e-7522-2816debbc367@kernel.org

arch/x86/kernel/head_32.S

@@ -23,6 +23,7 @@
 #include <asm/cpufeatures.h>
 #include <asm/percpu.h>
 #include <asm/nops.h>
+#include <asm/nospec-branch.h>
 #include <asm/bootparam.h>
 #include <asm/export.h>
 #include <asm/pgtable_32.h>