Fix windows adb build

libc++ is not available on windows yet, but it already defaults to
static libstdc++.

Change-Id: I85a766ead84f71fe1f2f59be6ac739b0b833b6db
diff --git a/adb/Android.mk b/adb/Android.mk
index 3c48585..d120f0a 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -165,7 +165,10 @@
     libcutils \
     $(EXTRA_STATIC_LIBS) \
 
-LOCAL_CXX_STL := libc++_static
+# libc++ not available on windows yet
+ifneq ($(HOST_OS),windows)
+    LOCAL_CXX_STL := libc++_static
+endif
 
 # Don't add anything here, we don't want additional shared dependencies
 # on the host adb tool, and shared libraries that link against libc++