Add missing string include from generatorprofile.h.
Add missing string include from generatorprofile.h.

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