ffmpeg_wrapper_install() should use drupal_get_filename()

In ffmpeg_wrapper.install on line 11, there's the following line:

require_once('./ffmpeg_wrapper.module');

This breaks in certain situations like when enabling the module via Drush because the working directory is not the ffmpeg_wrapper module directory. Instead, it should be:

require_once(drupal_get_filename('module', 'ffmpeg_wrapper'));

which will ensure ffmpeg_wrapper.module is loaded regardless of the current working directory. Attached is a patch.