powerpc/mm: Declare preload_new_slb_context() prototype

It fixes this W=1 compile error :

../arch/powerpc/mm/book3s64/slb.c:380:6: error: no previous prototype for ‘preload_new_slb_context’ [-Werror=missing-prototypes]
  380 | void preload_new_slb_context(unsigned long start, unsigned long sp)
      |      ^~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-15-clg@kaod.org

arch/powerpc/include/asm/book3s/64/mmu-hash.h

@@ -525,6 +525,7 @@ void slb_dump_contents(struct slb_entry *slb_ptr);
 
 extern void slb_vmalloc_update(void);
 extern void slb_set_size(u16 size);
+void preload_new_slb_context(unsigned long start, unsigned long sp);
 #endif /* __ASSEMBLY__ */
 
 /*