xfce4-panel GIT too much include cleanups cause things to not compile anymore

Hi,

looks like too much optimizing in GIT 😄

Please have a review and fix.

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  CC       libxfce4panel_2_0_la-xfce-panel-image.lo
xfce-panel-image.c: In function 'xfce_panel_image_scale_pixbuf':
xfce-panel-image.c:629:16: error: implicit declaration of function 'rint' [-Wimplicit-function-declaration]
  629 |   dest_width = rint (source_width * ratio);
      |                ^~~~
xfce-panel-image.c:32:1: note: include '<math.h>' or provide a declaration of 'rint'
   31 | #include <libxfce4util/libxfce4util.h>
  +++ |+#include <math.h>
   32 | 
xfce-panel-image.c:629:16: warning: incompatible implicit declaration of built-in function 'rint' [-Wbuiltin-declaration-mismatch]
  629 |   dest_width = rint (source_width * ratio);
      |                ^~~~
xfce-panel-image.c:629:16: note: include '<math.h>' or provide a declaration of 'rint'
make[6]: *** [Makefile:999: libxfce4panel_2_0_la-xfce-panel-image.lo] Error 1
make[6]: Leaving directory '/home/patronos/Documents/xfce4-panel/libxfce4panel'
make[5]: *** [Makefile:823: all] Error 2
make[5]: Leaving directory '/home/patronos/Documents/xfce4-panel/libxfce4panel'
make[4]: *** [Makefile:1335: distdir-am] Error 2
make[4]: Leaving directory '/home/patronos/Documents/xfce4-panel/libxfce4panel'
make[3]: *** [Makefile:1331: distdir] Error 2
make[3]: Leaving directory '/home/patronos/Documents/xfce4-panel/libxfce4panel'
make[2]: *** [Makefile:740: distdir-am] Error 1
make[2]: Leaving directory '/home/patronos/Documents/xfce4-panel'
make[1]: *** [Makefile:734: distdir] Error 2
make[1]: Leaving directory '/home/patronos/Documents/xfce4-panel'
make: *** [Makefile:842: dist] Error 2
/home/patronos/Documents

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