Fix exclude rule for scala-xml from scala-compiler

* build.sbt (libraryDependencies): There's no evidence that SBT
supports Ivy's exclude globbing.  See Ivy documentation
http://ant.apache.org/ivy/history/2.4.0/ivyfile/artifact-exclude.html

Author: ashawley

build.sbt

@@ -36,7 +36,7 @@ libraryDependencies += "org.scalacheck" %% "scalacheck" % "1.13.2" % "test"
 
 //// testing:
 // used in CompilerErrors test
-libraryDependencies += ("org.scala-lang" % "scala-compiler" % scalaVersion.value % "test").exclude("org.scala-lang.modules", s"scala-xml*")
+libraryDependencies += ("org.scala-lang" % "scala-compiler" % scalaVersion.value % "test").exclude("org.scala-lang.modules", s"scala-xml_${scalaVersion.value}")
 
 mimaPreviousVersion := Some("1.0.5")