meta-ibm/state: fix meson param statement

Change-Id: I4453f9b01f986a529c76710d665ab57639232b2d
Signed-off-by: Anton D. Kachalov <gmouse@google.com>
diff --git a/meta-ibm/recipes-phosphor/state/phosphor-state-manager_%.bbappend b/meta-ibm/recipes-phosphor/state/phosphor-state-manager_%.bbappend
index 4720112..5b35a0a 100644
--- a/meta-ibm/recipes-phosphor/state/phosphor-state-manager_%.bbappend
+++ b/meta-ibm/recipes-phosphor/state/phosphor-state-manager_%.bbappend
@@ -1,4 +1,4 @@
-EXTRA_OEMESON_witherspoon += "-Dwarm-reboot=disabled"
+EXTRA_OEMESON_append_witherspoon = " -Dwarm-reboot=disabled"
 
 # The scheduled-host-transition package provides support to
 # schedule power on and off operations for the host at some