Fix year range in unittests/.../char/empty.cc copyright header

Fix year range in unittests/.../char/empty.cc copyright header

The year range in the copyright header of that file was incorrect:

    // Copyright (C) 3 Free Software Foundation, Inc.

Tracing the origin of this file down, I found that it was copied
from GCC's libstdc++-v3 testsuite:

    Adapt and integrate string_view tests
    https://www.sourceware.org/ml/gdb-patches/2018-04/msg00113.html

Looking at the version in GCC, I found the same issue, so sent
a fix there:

    https://gcc.gnu.org/ml/gcc-patches/2019-01/msg00000.html

Now that the fix is in GCC, this commit applies the same fix
to our copy.

gdb/ChangeLog:

	* unittests/basic_string_view/element_access/char/empty.cc:
	Fix year range in copyright header.