Moves PHP4 constructor in BP_Groups_Widget below __construct()

This help to avoid a "constructor defined twice" strict standards warning in
PHP 5.4+.

See #5108

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7290 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Pinking shears

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7291 cdf35c40-ae34-48e0-9cc9-0c9da1808c22