powerpc/mm/64s: Fix slb_setup_new_exec() sparse warning

Sparse says:
  symbol slb_setup_new_exec was not declared. Should it be static?

No, it should have a declaration in a header, add one.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200916115637.3100484-1-mpe@ellerman.id.au

arch/powerpc/mm/book3s64/internal.h

@@ -13,4 +13,6 @@ static inline bool stress_slb(void)
 	return static_branch_unlikely(&stress_slb_key);
 }
 
+void slb_setup_new_exec(void);
+
 #endif /* ARCH_POWERPC_MM_BOOK3S64_INTERNAL_H */

arch/powerpc/mm/book3s64/mmu_context.c

@@ -21,6 +21,8 @@
 #include <asm/mmu_context.h>
 #include <asm/pgalloc.h>
 
+#include "internal.h"
+
 static DEFINE_IDA(mmu_context_ida);
 
 static int alloc_context_id(int min_id, int max_id)
@@ -48,8 +50,6 @@ int hash__alloc_context_id(void)
 }
 EXPORT_SYMBOL_GPL(hash__alloc_context_id);
 
-void slb_setup_new_exec(void);
-
 static int realloc_context_ids(mm_context_t *ctx)
 {
 	int i, id;