Build libnativehelper_header_only against NDK

Needed as some downstream targets that need to use these library
headers are themselves NDK builds.

Bug: 134379140
Test: m checkbuild
Change-Id: I8d91fac17ad0834293df1f2731103e486715d10b
diff --git a/Android.bp b/Android.bp
index f2340f1..f5a07b4 100644
--- a/Android.bp
+++ b/Android.bp
@@ -33,6 +33,7 @@
             enabled: true,
         },
     },
+    sdk_version: "current",
 }
 
 cc_library_headers {