open64: ensure that openat check works

For ARC builds atleast, __NR_xxx was not trickling into in open64.c
causing open64->openat a needless detour via open

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

libc/sysdeps/linux/common/open64.c

@@ -5,6 +5,7 @@
  */
 
 #include <_lfs_64.h>
+#include <sys/syscall.h>
 #include <fcntl.h>
 #include <stdarg.h>
 #include <cancel.h>