genpy: improving the run-time dependency

When importing the rosbag module in python, it fails in a genpy python module
complaining about missing the shutil python module. The shutils module is
provided by python-shell.

recipes-ros/genpy/genpy_0.4.10.bb

@@ -4,7 +4,7 @@ LICENSE = "BSD"
 LIC_FILES_CHKSUM = "file://package.xml;beginline=7;endline=7;md5=d566ef916e9dedc494f5f793a6690ba5"
 
 DEPENDS = "genmsg-native"
-RDEPENDS_${PN} = "python-math genmsg"
+RDEPENDS_${PN} = "python-math genmsg python-shell"
 RDEPENDS_${PN}_class-native = ""
 
 SRC_URI = "https://github.com/ros/${BPN}/archive/${PV}.tar.gz;downloadfilename=${BP}.tar.gz"