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.

Author: pablogsal

.github/CODEOWNERS

@@ -67,6 +67,10 @@ Include/pytime.h              @pganssle @abalkin
 **/*imap*                     @python/email-team
 **/*poplib*                   @python/email-team
 
+# Garbage collector
+/Modules/gcmodule.c           @pablogsal
+/Doc/library/gc.rst           @pablogsal
+
 # Parser/Pgen
 /Parser/pgen/                 @pablogsal