um, x86: Fix vDSO build

Commit 663b55b9b39 ("x86: Delete non-required instances of include <linux/init.h>")
broke the UML build.

  arch/x86/um/vdso/vdso.S: Assembler messages:
  arch/x86/um/vdso/vdso.S:2: Error: no such instruction: `__initdata'
  arch/x86/um/vdso/vdso.S:9: Error: no such instruction: `__finit'

UML's vDSO needs linux/init.h.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: user-mode-linux-devel@lists.sourceforge.net
Cc: paul.gortmaker@windriver.com
Link: http://lkml.kernel.org/r/1389538341-31383-1-git-send-email-richard@nod.at
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/um/vdso/vdso.S

@@ -1,3 +1,4 @@
+#include <linux/init.h>
 
 __INITDATA