liblibfdt_bindgen: Only use libfdt as header_libs

Only use the library for its headers, as the bindgen doesn't actually
require any object files.

Test: m pvmfw
Change-Id: I49885cd80d20ca99426f1be111da9a1c28d8885a
diff --git a/libs/libfdt/Android.bp b/libs/libfdt/Android.bp
index b2e7b2b..4ec0cc5 100644
--- a/libs/libfdt/Android.bp
+++ b/libs/libfdt/Android.bp
@@ -19,7 +19,7 @@
     dylib: {
         enabled: false,
     },
-    static_libs: [
+    header_libs: [
         "libfdt",
     ],
     apex_available: ["com.android.virt"],