[build] Fix make xa-all to build on Java.Interop source changes

Author: jonpryor

Makefile

@@ -70,7 +70,7 @@ bin/$(CONFIGURATION)/Android.Interop-Tests.dll: $(wildcard src/Android.Interop/*
 	$(XBUILD)
 	touch $@
 
-bin/$(XA_CONFIGURATION)/Java.Interop.dll:
+bin/$(XA_CONFIGURATION)/Java.Interop.dll: $(wildcard src/Java.Interop/*/*.cs)
 	$(XBUILD) /p:Configuration=$(XA_CONFIGURATION)
 
 CSHARP_REFS = \