Only prepend 'css/' when given a filename without path

Commit 1819bbdf8c2d629798fa48537f9bb167e8d33005 introduced new
html_css_link() function to include CSS files, but made the assumption
that these would always be in the css/ directory.

This lets the admin specify $g_css_include_file with a path, allowing
them to store custom CSS in a different location within the MantisBT
root.

Fixes 0021194