trivial: just avoid to hardcode the build directory that can potentially be modified in the pom

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/fileupload/trunk@1461190 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/src/main/assembly/bin.xml b/src/main/assembly/bin.xml
index 4b5ecc5..d28d5c7 100644
--- a/src/main/assembly/bin.xml
+++ b/src/main/assembly/bin.xml
@@ -32,7 +32,7 @@
       </includes>
     </fileSet>
     <fileSet>
-      <directory>target</directory>
+      <directory>${project.build.directory}</directory>
       <outputDirectory>lib</outputDirectory>
       <includes>
         <include>${project.build.finalName}.jar</include>