Define GNULIB_NAMESPACE in unittests/string_view-selftests.c

Define GNULIB_NAMESPACE in unittests/string_view-selftests.c

When building with x86_64-w64-mingw32-g++ (to test cross-compiling for
Windows), I get this error:

unittests/string_view-selftests.o: In function `selftests::string_view::inserters_2::test05(unsigned long long)':
/home/emaisin/src/binutils-gdb/gdb/unittests/basic_string_view/inserters/char/2.cc:60: undefined reference to `std::basic_ofstream<char, std::char_traits<char> >::rpl_close()'

This is caused by gnulib redefining "close" as "rpl_close", and
therefore messing up the declaration of basic_ofstream in the libstdc++
header.  The solution would be to use gnulib namespaces [1].  Until we
use them across GDB, we can use them locally in files that are
problematic, like this one.

gdb/ChangeLog:

	* unittests/string_view-selftests.c: Define GNULIB_NAMESPACE.