Add -keepattributes SourceFile to r8lib rules

Otherwise we cannot guarantee proper retracing of R8.

Change-Id: I04488239c8c7e0009d7e00b0f0d6b9b367307fcf
diff --git a/src/main/keep.txt b/src/main/keep.txt
index b0c58c9..2450f6b 100644
--- a/src/main/keep.txt
+++ b/src/main/keep.txt
@@ -19,7 +19,7 @@
 -keep public class com.android.tools.r8.Version { public static java.lang.String getPreReleaseString(); }
 -keep public class com.android.tools.r8.Version { public static boolean isDevelopmentVersion(); }
 
--keepattributes LineNumberTable, InnerClasses, EnclosingMethod, Exceptions, Signature
+-keepattributes SourceFile, LineNumberTable, InnerClasses, EnclosingMethod, Exceptions, Signature
 
 -keeppackagenames com.android.tools.r8