riscv: Add extern declarations for vDSO time-related functions

Add extern declarations for vDSO time-related functions to notify the
compiler these functions will be used in somewhere to avoid
"no previous prototype" compile warning.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/vdso/vgettimeofday.c

@@ -9,16 +9,22 @@
 #include <linux/time.h>
 #include <linux/types.h>
 
+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);
 }
 
+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);
 }
 
+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);