gdb: add IWYU export pragams to gdb_curses.h

It seems like gdb_curses.h is included whenever we want to access
ncurses functionality, instead of including directly ncurses.h.  As a
result, clangd often erroneously shows that gdb_curses.h inclusions are
unused.

By adding those pragmas, clangd (and the include-what-you-use tool)
understands that gdb_curses.h is a valid provider for whatever these
ncurses.h files provide.

Change-Id: Ia8acd761dae1577f7151d5fb558f35514b4e4ea2