suzerain: correct edge_u__x computation

I had incorrectly chained out the derivative of u
given derivatives of rhou and rho.  Fortunately, I think,
this impacted only the "bl.pg" status line.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@45166 0bd85f0b-c151-43b9-a1e9-27231104551f