Add flush statement after the search form. Searching can take a long time,

Add flush statement after the search form. Searching can take a long time,
this makes sure the search form is output as a whole directly when it's
available. With me, the search form will only be output partly as the
other half is buffered waiting for search results.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@6752 7612ce4b-ef26-0410-bec9-ea0150e637f0