fb_devel.install: Should not use drupal_get_path on hook_requirements

fb_devel.install uses drupal_get_path on line 78 (fb_devel.install,v 1.3 2010/03/20 19:44:14 yogadex)

drupal_get_path requires that drupal be already installed. However, hook_requirements is often executed prior to installing a profile. At that time the system table doesn't exist, which causes the installation to crash when checking requirements for the fb_devel module.