twl_init functions are declared in pm.h and used in pm.c
pm.h header defining the protos need to be included to
ensure that omap_twl.c has consistent function definition.
This fixes sparse warning:
arch/arm/mach-omap2/omap_twl.c:237:12: warning: symbol 'omap4_twl_init' was not declared. Should it be static?
arch/arm/mach-omap2/omap_twl.c:256:12: warning: symbol 'omap3_twl_init' was not declared. Should it be static?

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>