move and deduplicate declarations of __vdsosym to make it checkable

src/internal/syscall.h

@@ -246,4 +246,6 @@ hidden long __syscall_ret(unsigned long), __syscall(syscall_arg_t, ...),
 
 void __procfdname(char [static 15+3*sizeof(int)], unsigned);
 
+void *__vdsosym(const char *, const char *);
+
 #endif

src/sched/sched_getcpu.c

@@ -6,8 +6,6 @@
 
 #ifdef VDSO_GETCPU_SYM
 
-void *__vdsosym(const char *, const char *);
-
 static void *volatile vdso_func;
 
 typedef long (*getcpu_f)(unsigned *, unsigned *, void *);

src/time/clock_gettime.c

@@ -7,8 +7,6 @@
 
 #ifdef VDSO_CGT_SYM
 
-void *__vdsosym(const char *, const char *);
-
 static void *volatile vdso_func;
 
 static int cgt_init(clockid_t clk, struct timespec *ts)