Use file= fields for license tags in package.xml (UniversalRobots#63)

Author: fmauch

LICENSE LICENSE_apache_2_0

@@ -10,10 +10,10 @@
   <author>Lea Steffen</author>
   <author>Tristan Schnell</author>
   <maintainer email="[email protected]">Felix Exner</maintainer>
-  <license>Apache-2.0</license>
-  <license>BSD-2-Clause</license>
-  <license>Zlib</license>
-  <license>MIT</license>
+  <license file="LICENSE_apache_2_0">Apache-2.0</license>
+  <license file="include/ur_client_library/queue/LICENSE.md">BSD-2-Clause</license>
+  <license file="include/ur_client_library/queue/atomicops.h">Zlib</license>
+  <license file="tests/resources/dockerursim/Dockerfile">MIT</license>
 
   <url type="website">http://wiki.ros.org/ur_client_library</url>
   <url type="bugtracker">https://github.com/UniversalRobots/Universal_Robots_Client_Library/issues</url>