In groupNormalizationPlugin, the .cu files are not added to
PLUGIN_CU_SOURCES, contrary to other plugins.
This is problematic because the NVCC flags are not set correctly when
compiling those .cu files, e.g. GENCODES.
While this is not a direct issue for this plugin in particular, it
should be fixed as the documentation refers to it for building new
plugings.
Signed-off-by: Mickaël Seznec <mickael.seznec@gmail.com>