MAINTAINERS: add gnss scm tree

Add SCM tree for the gnss subsystem.
Signed-off-by: Johan Hovold <johan@kernel.org>

MAINTAINERS

@@ -6316,6 +6316,7 @@ F:	include/uapi/linux/gigaset_dev.h
 
 GNSS SUBSYSTEM
 M:	Johan Hovold <johan@kernel.org>
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/johan/gnss.git
 S:	Maintained
 F:	Documentation/ABI/testing/sysfs-class-gnss
 F:	Documentation/devicetree/bindings/gnss/