Change ENABLE_KSM to $ENABLE_KSM

* Since ENABLE_KSM param will be used in local.conf file
  and it's value is received in a variable and while compairing,
  the variable needs to be compared. So we need to change the
  same.

Change-Id: Id4ed17c0642acd2313e456503cfc375ca6f61409
Closes-Bug: #1724690
diff --git a/stack.sh b/stack.sh
index c545c56..f14ed96 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1006,7 +1006,7 @@
 # be memory bound not cpu bound so enable KSM by default but allow people
 # to opt out if the CPU time is more important to them.
 
-if [[ "ENABLE_KSM" == "True" ]] ; then
+if [[ $ENABLE_KSM == "True" ]] ; then
     if [[ -f /sys/kernel/mm/ksm/run ]] ; then
         sudo sh -c "echo 1 > /sys/kernel/mm/ksm/run"
     fi