MAINTAINERS: update the orphaned cpus-common.c file

We forgot to update MAINTAINERS when this code was re-factored.

Fixes: 267f685
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200513173200.11830-5-alex.bennee@linaro.org>

MAINTAINERS

@@ -115,6 +115,7 @@ M: Richard Henderson <rth@twiddle.net>
 R: Paolo Bonzini <pbonzini@redhat.com>
 S: Maintained
 F: cpus.c
+F: cpus-common.c
 F: exec.c
 F: accel/tcg/
 F: accel/stubs/tcg-stub.c