Fix to the "pmd" build target

PMD reports were being generated correctly when invoked from the
"reports" target but when run standalone the build directory was not being
created prior task execution.


git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@1536947 13f79535-47bb-0310-9956-ffa450edef68

build.xml

         <path refid="libs-tools-build-classpath"/>
       </classpath>
     </taskdef>
+    <mkdir dir="${build.dir}"/>
     <pmd shortFilenames="true" targetjdk="${javac.target}">
       <ruleset>basic</ruleset>
       <ruleset>rulesets/migrating_to_14.xml</ruleset>