Add libdexfiled_external to the debug Runtime APEX.

Follow up to http://r.android.com/943476. This library is only available in
debug builds of the Runtime APEX.

Test: Flash and boot with eng build
Test: adb shell ls apex/com.android.runtime/lib\*/libdexfiled_external.so
Change-Id: Ib6bbfacc19416ac1221a962f111b3937e61193e7
diff --git a/build/apex/Android.bp b/build/apex/Android.bp
index 95aea3c..82c8392 100644
--- a/build/apex/Android.bp
+++ b/build/apex/Android.bp
@@ -76,6 +76,7 @@
     "libadbconnectiond",
     "libartd",
     "libartd-compiler",
+    "libdexfiled_external",
     "libopenjdkjvmd",
     "libopenjdkjvmtid",
 ]