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>

Author: DragonBluep

arch/mips/include/asm/vpe.h

@@ -119,4 +119,12 @@ void cleanup_tc(struct tc *tc);
 
 int __init vpe_module_init(void);
 void __exit vpe_module_exit(void);
+
+#ifdef CONFIG_MIPS_VPE_LOADER_MT
+void *vpe_alloc(void);
+int vpe_start(void *vpe, unsigned long start);
+int vpe_stop(void *vpe);
+int vpe_free(void *vpe);
+#endif /* CONFIG_MIPS_VPE_LOADER_MT */
+
 #endif /* _ASM_VPE_H */