Add @pablogsal to code owners file for the garbage collector (GH-17248)

Add myself to the codeowners file as I would like to
be automatically added as a reviewer for PRs that touch
that component and its documentation.
diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index af5deea..0df7b5d 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -67,6 +67,10 @@
 **/*imap*                     @python/email-team
 **/*poplib*                   @python/email-team
 
+# Garbage collector
+/Modules/gcmodule.c           @pablogsal
+/Doc/library/gc.rst           @pablogsal
+
 # Parser/Pgen
 /Parser/pgen/                 @pablogsal