build: don't distribute generated coreutils.h

build: don't distribute generated coreutils.h

This issue was identified by the manifest comparisons
done by `make distcheck`

* src/local.mk (noinst_HEADERS): Remove coreutils.h from this always
distributed list.
(nodist_src_coreutils_SOURCES): Add coreutils.h as its contents
are determined at configure time, so pointless to distribute.
(src_coreutils_SOURCES): Define explicitly so that the corresponding
nodist_ variable is honored.
(DISTCLEANFILES): Add coreutils.h to this rather than CLEANFILES,
as its contents are determined at configure time.