Add a changelog entry.

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+Removed: The include files in `include/deal.II/base/std_cxx1x`,
+deprecated since 2014, have been removed. They had introduced a
+namespace `std_cxx1x` at a time when deal.II did not yet require
+C++11-compatible compilers, and this namespace was later also made
+available via the name `std_cxx11` once C++11 was actually
+published. In any case, none of this is necessary any more since we
+now have C++11 compilers.
+<br>
+(Wolfgang Bangerth, 2019/12/02)