1) atan2() returns radians, so an integer wouldn't have been of much use anyway.
2) rotate_sprite() uses Allegro degrees, which are *128/π to the radian, meaning even if you would have used a float and ftofix it wouldn't have worked the way you wanted it to.
3) The conflict is because you've just met Allegro's Dreaded Fix Class^TM, which overloads the regular trig functions. Include <math.h> before <allegro.h> to scare it off.

[EDIT]
But if you're sure you actually do want to used fixed point (honestly, there's really no need for them on any non-ancient machine), skip the evil overload.
fixed angle = fixatan2(mouse_y, mouse_x);