Regression from commit 54a2c8c0
(tdf#39080 Interactive hide-whitespace mode, 2015-08-30), the problem
was that SwPageFrm::IsOverHeaderFooterArea() checked if the header is
active before accepting that the user clicked in that area.

Don't require an active header at least in non-hide-whitespace mode, as
the member function is used in the "could there be a header here" sense
in SwEditWin::MouseButtonDown() to show the controls that allows
actually adding a header.

Change-Id: I6f905920113aed1512e333e718a8f26d88a3245d
(cherry picked from commit a23f85f1)