x86/vdso: Provide missing include file

When building with C=1, sparse issues a warning:

  CHECK   arch/x86/entry/vdso/vdso32-setup.c
  arch/x86/entry/vdso/vdso32-setup.c:28:28: warning: symbol 'vdso32_enabled' was not declared. Should it be static?

Provide the missing header file.
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/36224.1575599767@turing-police

arch/x86/entry/vdso/vdso32-setup.c

@@ -11,6 +11,7 @@
 #include <linux/smp.h>
 #include <linux/kernel.h>
 #include <linux/mm_types.h>
+#include <linux/elf.h>
 
 #include <asm/processor.h>
 #include <asm/vdso.h>