BUG/BUILD: opentracing: fixed OT_DEFINE variable setting

In case of using parameter 'OT_USE_VARS=1', the value of the OT_DEFINE
variable is set incorrectly (i.e. the previous value was deleted and a
new one set instead of adding new content).

This patch must be backported in 2.5.
diff --git a/addons/ot/Makefile b/addons/ot/Makefile
index c7b0f8d..18edd0a 100644
--- a/addons/ot/Makefile
+++ b/addons/ot/Makefile
@@ -67,7 +67,7 @@
 ifneq ($(OT_USE_VARS),)
 $(warning Please do not set the OT_USE_VARS configuration variable, as the source will probably not be able to compile!  For now, this variable can only be used for experimental purposes, and is not intended for wider use.)
 
-OT_DEFINE     = -DUSE_OT_VARS
+OT_DEFINE    += -DUSE_OT_VARS
 OPTIONS_OBJS += \
 	addons/ot/src/vars.o
 endif