c6x: fix build failure caused by cache.h

A patch to linux/irqflags.h uncovered a problem with c6x asm/cache.h
which causes a build failure:

/arch/c6x/include/asm/cache.h:63:20: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘c6x_cache_init’
 extern void __init c6x_cache_init(void);

The asm/cache.h was relying on linux/irqflags.h to pull in linux/init.h
but the recent patch changed that. The c6x header should have included
linux/init.h all along.

Signed-off-by: Mark Salter <msalter@redhat.com>