macosx: create_dmg: also install the C language IRMP shared library

Have the install_name_tool also massage the libirmp.la build output and
thus have the shared library added to the resulting package. It was
missing before and the Python part of IR_IRMP failed to load.

Add a comment for developers' awareness, the application's executable
and the data files are handled in a different section of the script than
the shared libraries were. Drop trailing whitespace from a text line.