ARM: OMAP2+: mux: fix sparse warning

Commit bbd707ac ("ARM: omap2: use
machine specific hook for late init") resulted in the addition of this
sparse warning:

arch/arm/mach-omap2/mux.c:791:12: warning: symbol 'omap_mux_late_init' was not declared. Should it be static?

Fix by including the header file containing the prototype.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Tony Lindgren <tony@atomide.com>