common.mk: clean miniprelude.c

* common.mk (realclean-local): miniprelude.c is made by srcs, so it
  should not removed by distclean but by realclean.  [Bug #6807]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39492 b2dd03c8-39d4-4d8f-98ff-823fe69b080e