Readd TARGET_H to the list of exported plugin headers, fixing PR plugins/56754.
TARGET_H was removed in commit 188166, however several plugins need it, see the
PR for details.  Applying under the trivial rule.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@199144 138bc75d-0d04-0410-961f-82ee72b054a4