um: Fix header inclusion

User Mode Linux is a flavor of x86 that from the vDSO prospective always
falls back on system calls. This implies that it does not require any
of the unified vDSO definitions and their inclusion causes side effects
like this:

In file included from include/vdso/processor.h:10:0,
from include/vdso/datapage.h:17,
from arch/x86/include/asm/vgtod.h:7,
from arch/x86/um/../kernel/sys_ia32.c:49:
>> arch/x86/include/asm/vdso/processor.h:11:29: error: redefinition of 'rep_nop'
static __always_inline void rep_nop(void)
^~~~~~~
In file included from include/linux/rcupdate.h:30:0,
from include/linux/rculist.h:11,
from include/linux/pid.h:5,
from include/linux/sched.h:14,
from arch/x86/um/../kernel/sys_ia32.c:25:
arch/x86/um/asm/processor.h:24:20: note: previous definition of 'rep_nop' was here
static inline void rep_nop(void)

Make sure that the unnecessary headers are not included when um is built
to address the problem.

Fixes: abc22418db02 ("x86/vdso: Enable x86 to use common headers")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200323124109.7104-1-vincenzo.frascino@arm.com