Add equation of state directory to the Doxygen documentation.

doc/Doxyfile.in

@@ -765,6 +765,7 @@ INPUT		       += @top_srcdir@/src/gravity/Default
 INPUT		       += @top_srcdir@/src/stars/Default
 INPUT		       += @top_srcdir@/src/riemann
 INPUT		       += @top_srcdir@/src/potential/point_mass
+INPUT		       += @top_srcdir@/src/equation_of_state/ideal_gas
 INPUT		       += @top_srcdir@/src/cooling/none
 INPUT		       += @top_srcdir@/src/chemistry/none