MAINTAINERS: Add Gunnar as a maintainer

Gunnar's commits to bmcweb can be found at:
https://github.com/openbmc/bmcweb/commits?author=gtmills

Change-Id: I7747569fcf4d54e1e657e11148315bc50178188c
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 1c252d1..229c9d3 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -45,3 +45,4 @@
 M:  James Feist <james.feist@linux.intel.com> <jfei!>
 M:  Vernon Mauery <vernon.mauery@linux.intel.com> <vmauery!>
 M:  Jason Bills <jason.m.bills@linux.intel.com> <jmbills!>
+M:  Gunnar Mills <gmills@linux.vnet.ibm.com> <gunnarm!>