Build hostdex on only Linux.

Change-Id: I9213684e4c65632c54e9929f693f4b6a24beb95a
diff --git a/core/host_dalvik_java_library.mk b/core/host_dalvik_java_library.mk
index 061511b..e946b24 100644
--- a/core/host_dalvik_java_library.mk
+++ b/core/host_dalvik_java_library.mk
@@ -20,6 +20,7 @@
 # They will be compiled against libcore and not the host JRE.
 #
 
+ifeq ($(HOST_OS),linux)
 USE_CORE_LIB_BOOTCLASSPATH := true
 
 #######################################
@@ -108,3 +109,5 @@
 	$(add-dex-to-package)
 
 USE_CORE_LIB_BOOTCLASSPATH :=
+
+endif