consistent constness for getFunctionSymbol() overloads
consistent constness for getFunctionSymbol() overloads

This is an inline function, so there should be no problems with binary
compatibility.

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core