Bug 31974: Regression: Bug 31813 incorrectly affected labels in the header search

Bug 31974: Regression: Bug 31813 incorrectly affected labels in the header search

This patch adds a specific white-space property to header search labels
so that they do not wrap. The change made by Bug 31813 overrode the
white-space property incorrectly.

To test, apply the patch and rebuild the staff interface CSS.

- Go to Administration.
- The header search label "Search system preferences" should appear on
  one line.
- Check that labels still wrap correctly on pages like the patron edit
  screen and the staged MARC record detail view.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>