Fix another build failure in tui-selftests.c

Fix another build failure in tui-selftests.c

Christian had emailed me to say that the TUI unit test broke the mingw
build, but I erroneously thought this was fixed by the earlier patch
that made the test body conditional on the TUI being built.

However, I was wrong about this -- tui-selftests.c unconditionally
includes tui-winsource.h, which fails if curses is not available.

This patch fixes the build problem by moving this include into the
"#ifdef TUI" section.

Tested by rebuilding a mingw-hosted gdb.

gdb/ChangeLog
2019-12-04  Tom Tromey  <tromey@adacore.com>

	* unittests/tui-selftests.c: Conditionally include tui-winsource.h.

Change-Id: If608649ef5cbef8ea92192e11c53379742967ee7