No using inside header files

I thought it was OK to use "using" in a header file so long as it was
inside a "namespace" block, but it just imports symbols from one
namespace into another, so things that shouldn't work do.

Test: Treehugger
Change-Id: I4d43d35339636af7e95761cada7120b4db638c01
diff --git a/MetadataCrypt.cpp b/MetadataCrypt.cpp
index dc50679..b1c5afc 100644
--- a/MetadataCrypt.cpp
+++ b/MetadataCrypt.cpp
@@ -49,6 +49,7 @@
 
 using android::fs_mgr::FstabEntry;
 using android::fs_mgr::GetEntryForMountPoint;
+using android::fscrypt::GetFirstApiLevel;
 using android::vold::KeyBuffer;
 using namespace android::dm;