syscall() declaration belongs in unistd.h, not sys/syscall.h

traditionally, both BSD and GNU systems have it this way.
sys/syscall.h is purely syscall number macros. presently glibc exposes
the syscall declaration in unistd.h only with _GNU_SOURCE, but that
does not reflect historical practice.

include/sys/syscall.h

@@ -1,16 +1,6 @@
 #ifndef _SYS_SYSCALL_H
 #define _SYS_SYSCALL_H
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-#include <features.h>
-
-long syscall(long, ...);
 
 #include <bits/syscall.h>
 
-#ifdef __cplusplus
-}
-#endif
 #endif