Try with include math.h for ubuntu build

Author: LinusDierheimer

src/logo/sixel/sixel.h

@@ -10,9 +10,7 @@ bool ffLogoPrintSixelIM7(FFinstance* instance);
 #endif
 
 #ifdef FF_HAVE_IMAGEMAGICK6
-#ifndef isnan
-    #define isnan(x) ((x) != (x))
-#endif
+#include <math.h>
 bool ffLogoPrintSixelIM6(FFinstance* instance);
 #endif