Set a dummy CM_PLATFORM_SDK_VERSION if it doesn't exist

* Avoids printing 2 build warnings on every single rebuild
  of anything when building TWRP inline

Change-Id: I5dd3623707c03af0dfe7366f1a77e83762e720b2
diff --git a/Android.mk b/Android.mk
index 6eb1cbc..9ad3507 100755
--- a/Android.mk
+++ b/Android.mk
@@ -34,6 +34,10 @@
     BOARD_SEPOLICY_UNION += twrp.te
 endif
 
+ifeq ($(CM_PLATFORM_SDK_VERSION),)
+    CM_PLATFORM_SDK_VERSION := 0
+endif
+
 include $(CLEAR_VARS)
 
 TWRES_PATH := /twres/