theme file key ignored when registering theme functions

In theory, a style plugin should support both theme functions and templates, in whatever file is specified by the style plugin's 'theme file' key. However, due to an oops in views_theme() it doesn't do a require_once() before checking for the theme function, so a function never gets registered if it doesn't happen to have already been included.

merlinofchaos said the relevant loop was line 53 of views.module and following.