LoongArch: Fix build warnings in VDSO

Fix build warnings in VDSO as below:

arch/loongarch/vdso/vgettimeofday.c:9:5: warning: no previous prototype for '__vdso_clock_gettime' [-Wmissing-prototypes]
    9 | int __vdso_clock_gettime(clockid_t clock,
      |     ^~~~~~~~~~~~~~~~~~~~
arch/loongarch/vdso/vgettimeofday.c:15:5: warning: no previous prototype for '__vdso_gettimeofday' [-Wmissing-prototypes]
   15 | int __vdso_gettimeofday(struct __kernel_old_timeval *tv,
      |     ^~~~~~~~~~~~~~~~~~~
arch/loongarch/vdso/vgettimeofday.c:21:5: warning: no previous prototype for '__vdso_clock_getres' [-Wmissing-prototypes]
   21 | int __vdso_clock_getres(clockid_t clock_id,
      |     ^~~~~~~~~~~~~~~~~~~
arch/loongarch/vdso/vgetcpu.c:27:5: warning: no previous prototype for '__vdso_getcpu' [-Wmissing-prototypes]
   27 | int __vdso_getcpu(unsigned int *cpu, unsigned int *node, struct getcpu_cache *unused)
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/vdso/vgetcpu.c

@@ -24,6 +24,8 @@ static __always_inline const struct vdso_pcpu_data *get_pcpu_data(void)
 	return (struct vdso_pcpu_data *)(get_vdso_base() - VDSO_DATA_SIZE);
 }
 
+extern
+int __vdso_getcpu(unsigned int *cpu, unsigned int *node, struct getcpu_cache *unused);
 int __vdso_getcpu(unsigned int *cpu, unsigned int *node, struct getcpu_cache *unused)
 {
 	int cpu_id;

arch/loongarch/vdso/vgettimeofday.c

@@ -6,20 +6,23 @@
  */
 #include <linux/types.h>
 
-int __vdso_clock_gettime(clockid_t clock,
-			 struct __kernel_timespec *ts)
+extern
+int __vdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts);
+int __vdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts)
 {
 	return __cvdso_clock_gettime(clock, ts);
 }
 
-int __vdso_gettimeofday(struct __kernel_old_timeval *tv,
-			struct timezone *tz)
+extern
+int __vdso_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz);
+int __vdso_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz)
 {
 	return __cvdso_gettimeofday(tv, tz);
 }
 
-int __vdso_clock_getres(clockid_t clock_id,
-			struct __kernel_timespec *res)
+extern
+int __vdso_clock_getres(clockid_t clock_id, struct __kernel_timespec *res);
+int __vdso_clock_getres(clockid_t clock_id, struct __kernel_timespec *res)
 {
 	return __cvdso_clock_getres(clock_id, res);
 }