Fix include directory usage requirement for build tree

The include directory usage requirement for the build tree was added by
commit 993f0ea4 (Add an option to split object and interface libs,
2017-06-19) and preserved by commit b61faf89 (Fix install rules and
usage requirements for interface+object libs, 2017-06-28).  However, its
value should be that of `KWSYS_HEADER_ROOT` rather than just its
default.