Fix mac build.

Do not build the bit tool on darwin.

Test: Make sure this still builds on linux.

Change-Id: Ie958649893016771f1fa3df6f10d443d0117c171
diff --git a/tools/bit/Android.mk b/tools/bit/Android.mk
index 7f691a0..1c1291f 100644
--- a/tools/bit/Android.mk
+++ b/tools/bit/Android.mk
@@ -22,6 +22,9 @@
 
 LOCAL_MODULE := bit
 
+# This tool doesn't build on darwin.
+LOCAL_MODULE_HOST_OS := linux
+
 LOCAL_SRC_FILES := \
     aapt.cpp \
     adb.cpp \