Make not having Jack non-fatal.

Not every build needs Java.

Bug: http://b/17441393
Change-Id: Ia55742cfd43c3a4f9c7873b87816bcce99f8156f
diff --git a/core/main.mk b/core/main.mk
index 27b559f..2bd3163 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -316,8 +316,8 @@
 #
 # -----------------------------------------------------------------
 # Jack version configuration
-include $(TOPDIR)prebuilts/sdk/tools/jack_versions.mk
-include $(TOPDIR)prebuilts/sdk/tools/jack_for_module.mk
+-include $(TOPDIR)prebuilts/sdk/tools/jack_versions.mk
+-include $(TOPDIR)prebuilts/sdk/tools/jack_for_module.mk
 
 # -----------------------------------------------------------------
 ###