vdso: Fix clocksource.h macro detection

CONFIG_GENERIC_GETTIMEOFDAY is a sufficient condition to verify if an
architecture implements asm/vdso/clocksource.h or not. The current
implementation wrongly assumes that the same is true for the config
option CONFIG_ARCH_CLOCKSOURCE_DATA.

This results in a series of build errors on ia64/sparc/sparc64 like this:

  In file included from ./include/linux/clocksource.h:31,
                   from ./include/linux/clockchips.h:14,
                   from ./include/linux/tick.h:8,
                   from fs/proc/stat.c:15:
  ./include/vdso/clocksource.h:9:10: fatal error: asm/vdso/clocksource.h:
  No such file or directory
      9 | #include <asm/vdso/clocksource.h>
        |          ^~~~~~~~~~~~~~~~~~~~~~~~

Fix the issue removing the unneeded config condition.

Fixes: 14ee2ac6 ("linux/clocksource.h: Extract common header for vDSO")
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200323133920.46546-1-vincenzo.frascino@arm.com