cmstart.sh: exec xcompmgr (#149)

This will release resources used by /bin/sh process

usr/lib/raspi-config/cmstart.sh

@@ -1,4 +1,4 @@
 #!/bin/sh
 if grep -q okay /proc/device-tree/soc/v3d@7ec00000/status 2> /dev/null || grep -q okay /proc/device-tree/soc/firmwarekms@7e600000/status 2> /dev/null ; then
-    xcompmgr -aR
+    exec xcompmgr -aR
 fi