Don't define _POSIX_SHARED_MEMORY_OBJECTS.

This isn't implemented, so don't define it.

system/lib/libc/musl/readme.txt

@@ -5,3 +5,4 @@ Differences from upstream musl include:
 * file descriptor numbers for stdio are 1-3 rather than 0-2.
 * various 64 bit types are 32 bit instead including off_t,
   ino_t, dev_t, blkcnt_t, fsblkcnt_t, fsfilcnt_t, rlim_t.
+* We don't define _POSIX_SHARED_MEMORY_OBJECTS.