Fix a regression in the layout_api.php file

Fixes 0035493.

When the nav-tabs element is displayed, the active area of the project
selection buttons (displayed when $g_show_project_menu_bar = ON) is
overlapped because nav-tabs has relative positioning and is attached
to the top of the parent element.

Introduced in https://github.com/mantisbt/mantisbt/pull/2096/commits/4395fad8d05e3808f7820993ae42bea5f7b35eaf

Signed-off-by: raspopov raspopov@cherubicsoft.com