Added a missing API header file.
Added a missing API header file.

MichaelClerx/libcellml,hsorby/libcellml,cellml/libcellml,MichaelClerx/libcellml,cellml/libcellml,nickerso/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml,MichaelClerx/libcellml,nickerso/libcellml,nickerso/libcellml,hsorby/libcellml,cellml/libcellml,cellml/libcellml