thunar-tpa: Fix libxfce4panel include
Include guards are added to headers other than libxfce4panel.h in libxfce4panel 4.21.0, so no headers other than this one should be included.
Code preceding this patch can still be built by linking against libxfce4panel >= 4.21.0 by adding -DLIBXFCE4PANEL_COMPILATION to CFLAGS.
Related: xfce4-panel!162 (merged)