gdb: unconditionally define _initialize_string_view_selftests

gdb: unconditionally define _initialize_string_view_selftests

The build failure was noticed by Helmut Jarausch in
https://bugs.gentoo.org/680232:
    $ ./configure CXXFLAGS='-std=c++17 -Os'
    ...
      CXXLD  gdb
    ld: init.o: in function `initialize_all_files()':
    init.c:(.text+0x113): undefined reference to `_initialize_string_view_selftests()'

It happens because '_initialize_string_view_selftests()' is
conditionally defined based on C++ default.

The change defines '_initialize_string_view_selftests()'
unconditionally and leaves implementation a no-op on c++17
compilers.

gdb/ChangeLog:
2019-03-17  Sergei Trofimovich <siarheit@google.com>

	* unittests/string_view-selftests.c: Define
	_initialize_string_view_selftests unconditionally.