Fix compilation of special functions without C99 math.

The commit with Bessel functions i0e and i1e placed the ifdef/endif incorrectly,
causing i0e/i1e to be undefined when EIGEN_HAS_C99_MATH=0. These functions do not
actually require C99 math, so now they are always available.

unsupported/Eigen/src/SpecialFunctions/SpecialFunctionsImpl.h

@@ -1574,6 +1574,8 @@ struct betainc_impl<double> {
   }
 };
 
+#endif  // EIGEN_HAS_C99_MATH
+
 /****************************************************************************
  * Implementation of Bessel function, based on Cephes                       *
  ****************************************************************************/
@@ -1902,8 +1904,6 @@ struct i1e_impl<double> {
   }
 };
 
-#endif  // EIGEN_HAS_C99_MATH
-
 }  // end namespace internal
 
 namespace numext {