Sorry, most efficient way with out stuffing your proj file is.
rebuild.
Congratulate your self for being smarter than everyone else and not ****ing you project. For me this exercise took under 10 seconds. Where as manually trying to input the compile... line into the csproj not only can render your project unusable but it is also impossible to maintain on large scale application. Better to keep source version control software to do the updates. If you need to cross merge branches then doing the above is amazing :).