CONFIG_GENERIC_GETTIMEOFDAY is a sufficient condition to verify if an
architecture implements or not asm/vdso/clocksource.h. The current
implementation wrongly assumes that the same is true for the config
option CONFIG_ARCH_CLOCKSOURCE_DATA.
This results in a series of building errors on ia64/sparc/sparc64 like
the one below:

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>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>