ARM: OMAP: 32k counter: resolve sparse warnings

Commit 1fe97c8f ("ARM: OMAP: Make OMAP
clocksource source selection using kernel param") results in a new warning
from sparse:

arch/arm/plat-omap/counter_32k.c:86:12: warning: symbol 'omap_init_clocksource_32k' was not declared. Should it be static?

This second version fixes this warning by including <plat/counter-32k.h>,
at Tony's request.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Vaibhav Hiremath <hvaibhav@ti.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Tony Lindgren <tony@atomide.com>