MIPS: Add prototypes for plat_post_relocation() and relocate_kernel()

When building malta_defconfig with CONFIG_RELOCATABLE=y, there are two
warnings due to missing prototypes for functions only used when that
configuration is enabled:

  arch/mips/kernel/relocate.c:42:12: warning: no previous prototype for 'plat_post_relocation' [-Wmissing-prototypes]
     42 | int __weak plat_post_relocation(long offset)
        |            ^~~~~~~~~~~~~~~~~~~~
  arch/mips/kernel/relocate.c:324:14: warning: no previous prototype for 'relocate_kernel' [-Wmissing-prototypes]
    324 | void *__init relocate_kernel(void)
        |              ^~~~~~~~~~~~~~~

While relocate_kernel() is only called from assembly, it makes sense to
keep the prototypes together in C to fix the warnings. Add them to
silence the warnings.
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/setup.h

@@ -2,6 +2,7 @@
 #ifndef _MIPS_SETUP_H
 #define _MIPS_SETUP_H
 
+#include <linux/init.h>
 #include <linux/types.h>
 #include <uapi/asm/setup.h>
 
@@ -29,4 +30,9 @@ extern void per_cpu_trap_init(bool);
 extern void cpu_cache_init(void);
 extern void tlb_init(void);
 
+#ifdef CONFIG_RELOCATABLE
+extern void * __init relocate_kernel(void);
+extern int plat_post_relocation(long);
+#endif
+
 #endif /* __SETUP_H */