add missing build_depend

As they are find_package(... REQUIRED) in the CMakeLists.txt
to fill in the catkin_package(DEPENDS ...) section,
they are build dependencies.

pcl_conversions/package.xml

@@ -18,6 +18,9 @@
 
   <buildtool_depend>catkin</buildtool_depend>
 
+  <build_depend>libpcl-all-dev</build_depend>
+  <build_depend>eigen</build_depend>
+
   <build_export_depend>eigen</build_export_depend>
   <build_export_depend>libpcl-all-dev</build_export_depend>
   <build_export_depend>pcl_msgs</build_export_depend>