clocksource: Explicitly include linux/clocksource.h when needed

The kbuild test robot reported errors in these files when doing an ia64
allmodconfig build.

drivers/clocksource/timer-sun5i.c:52:21: error: field 'clksrc' has incomplete type
   struct clocksource clksrc;
                      ^~~~~~
drivers/clocksource/cadence_ttc_timer.c:92:21: error: field 'cs' has incomplete type
   struct clocksource cs;
                      ^~

(and many more errors for these files)

Cc: Michal Simek <michal.simek@xilinx.com>
Cc: "Sören Brinkmann" <soren.brinkmann@xilinx.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>