add some missing macros to sys/shm.h

these are not specified in the standard, but in the reserved
namespace, so there is no problem with defining them unconditionally.

include/sys/shm.h

@@ -16,6 +16,9 @@ extern "C" {
 #include <sys/ipc.h>
 #include <bits/shm.h>
 
+#define SHM_R 0400
+#define SHM_W 0200
+
 #define SHM_RDONLY 010000
 #define SHM_RND    020000
 #define SHM_REMAP  040000