build: fix dependencies of man/sha*sum.1
Although the above man pages depend on src/md5sum.c as a shared
source, the build of the man pages directly requires their own
executables to exist.
* man/local.mk (man/sha1sum.1): Change the dependency from
'src/md5sum' to 'src/sha1sum'.
(man/sha224sum.1): s/md5sum/sha224sum/
(man/sha256sum.1): s/md5sum/sha256sum/
(man/sha384sum.1): s/md5sum/sha384sum/
(man/sha512sum.1): s/md5sum/sha512sum/
Reported by Pádraig Brady in
http://lists.gnu.org/archive/html/coreutils/2013-11/msg00006.html