scons: Update for builtin_stubs.cpp

scons: Update for builtin_stubs.cpp

Note this by itself is not enough to fix scons build -- it will fail
until you remove:

   rm -rf build/*/glsl/builtin_compiler

because that node was a filei before, but it will be now a directory.

This also means that bisecting across this change will require wiping
the build directory..