MIPS: Silence missing prototype warning

Silence the following two warnings when make W=1:

  CC      arch/mips/lib/bswapsi.o
arch/mips/lib/bswapsi.c:5:22: warning: no previous prototype for '__bswapsi2' [-Wmissing-prototypes]
 unsigned int notrace __bswapsi2(unsigned int u)
                      ^~~~~~~~~~
  CC      arch/mips/lib/bswapdi.o
arch/mips/lib/bswapdi.c:5:28: warning: no previous prototype for '__bswapdi2' [-Wmissing-prototypes]
 unsigned long long notrace __bswapdi2(unsigned long long u)
                            ^~~~~~~~~~
  AR      arch/mips/lib/built-in.a

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>