webdav: keep phodav requirements inside USE_PHODAV
webdav: keep phodav requirements inside USE_PHODAV

Commit 33d4016228798108250b0ef9173f2c4c719a5065 uses
g_output_stream_write_all_async/finish which is on gio 2.44 version.
This breaks the build on older systems even if --disable-webdav is used.

Using #USE_PHODAV to wrap those functions that will not be called
without --enable-webdav.

fgouget/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk