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/unistd.h

@@ -175,6 +175,7 @@ void setusershell(void);
 void endusershell(void);
 char *getusershell(void);
 int acct(const char *);
+long syscall(long, ...);
 #endif
 
 #ifdef _GNU_SOURCE