Tidy header inclusions in src/entity.cpp.
Tidy header inclusions in src/entity.cpp.

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