[libc] Add myself as maintainer for Public Headers / hdrgen

Author: frobtech

libc/Maintainers.rst

@@ -54,4 +54,9 @@ RISC-V
 | Mikhail R. Gadelha
 | mikhail\@igalia.com (email), `mikhailramalho <https://github.com/mikhailramalho>`_ (github)
 
+Public Headers / hdrgen
+------
+| Roland McGrath
+| mcgrathr\@google.com (email), `frobtech <https://github.com/frobtech>`_ (github)
+
 .. TODO: add "Inactive Maintainers" section when needed.