MAINTAINERS: add myself as a POSIX collaborator

Add myself as a collaborator in the POSIX subsystem.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

MAINTAINERS.yml

diff --git a/MAINTAINERS.yml b/MAINTAINERS.yml
index 1dffa8e..ef82c8e 100644
--- a/MAINTAINERS.yml
+++ b/MAINTAINERS.yml
@@ -2730,6 +2730,8 @@
   status: maintained
   maintainers:
     - cfriedt
+  collaborators:
+    - ycsin
   files:
     - include/zephyr/posix/
     - lib/posix/