MAINTAINERS: update ClangBuiltLinux IRC chat

Everyone has moved from Freenode to Libera so updated the channel entry
for MAINTAINERS.

Link: https://github.com/ClangBuiltLinux/linux/issues/1402
Link: https://lkml.kernel.org/r/20210818022339.3863058-1-nathan@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>