rtl/string.hxx and rtl/ustring.hxx both unnecessarily #include <sal/log.hxx>
(and don't make use of it themselves), but many other files happen to depend on
it.  Cleaned up some, but something like

  grep -FwL sal/log.hxx $(git grep -Elw \
    'SAL_INFO|SAL_INFO_IF|SAL_WARN|SAL_WARN_IF') -- \*.cxx)

shows lots more files that potentially need fixing before the include can be
removed from rtl/string.hxx and rtl/ustring.hxx.

Change-Id: Ibf033363e83d37851776f392dc0b077381cd8b90