Platform: correct posix_spawn API nullability

The `posix_spawn` API is not annotated appropriately for nullability.
Adjust the signature to allow proper usage with older NDKs.

Author: compnerd

stdlib/public/Platform/spawn.apinotes

@@ -5,3 +5,9 @@ Functions:
   Parameters:
   - Position: 0
     Type: "posix_spawnattr_t _Nullable * _Nonnull"
+- Name: posix_spawn
+  Parameters:
+  - Position: 2
+    Type: "const posix_spawn_file_actions_t _Nonnull * _Nullable"
+  - Position: 3
+    Type: "const posix_spawnattr_t _Nonnull * _Nullable"