MIPS: vpe-mt: add missing prototypes for vpe_{alloc,start,stop,free}

MIPS: vpe-mt: add missing prototypes for vpe_{alloc,start,stop,free}

These functions are exported but their prototypes are not defined.
This patch adds the missing function prototypes to fix the following
compilation warnings:

arch/mips/kernel/vpe-mt.c:180:7: error: no previous prototype for 'vpe_alloc' [-Werror=missing-prototypes]
  180 | void *vpe_alloc(void)
      |       ^~~~~~~~~
arch/mips/kernel/vpe-mt.c:198:5: error: no previous prototype for 'vpe_start' [-Werror=missing-prototypes]
  198 | int vpe_start(void *vpe, unsigned long start)
      |     ^~~~~~~~~
arch/mips/kernel/vpe-mt.c:208:5: error: no previous prototype for 'vpe_stop' [-Werror=missing-prototypes]
  208 | int vpe_stop(void *vpe)
      |     ^~~~~~~~
arch/mips/kernel/vpe-mt.c:229:5: error: no previous prototype for 'vpe_free' [-Werror=missing-prototypes]
  229 | int vpe_free(void *vpe)
      |     ^~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>