BHV-2417: Set moon.Icons to center text-align when inside moon.Header client area

There was a change to Header.less which assigned all children of a moon-header-client element to be "start" aligned. moon.Buttons had an explicit override rule to allow them to retain their centered alignment. Adding moon-icon to this rule-set extended this to moon.Icon and moon.IconButton kinds.

Enyo-DCO-1.1-Signed-off-by: Blake Stephens <blake.stephens@lge.com>