misc: Add 'stdlib' tag to MAINTAINERS.yaml

This tag is for the "gem5 standard library" which can be found in
`src/python/gem5`.

Change-Id: Idf276635cd3f1d729cfbb4b7195b20fbdddd6584
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51791
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

MAINTAINERS.yaml

@@ -201,6 +201,13 @@ learning-gem5:
   maintainers:
     - Jason Lowe-Power <jason@lowepower.com>
 
+stdlib:
+  desc: >-
+    The gem5 standard library found under `src/python/gem5`
+  status: maintained
+  maintainers:
+    - Bobby R. Bruce <bbruce@ucdavis.edu>
+
 mem:
   desc: >-
     General memory system (e.g., XBar, Packet)