Rename header.pot to pot.header.

The Translation Project has asked us to rename the
pot header file `header.pot' to something else. Their
scripts automatically look for pot files and the
file `header.pot' is not actually a pot file but a
header that we use when regenerating `libc.pot.'
This commit renames `header.pot' to `pot.header' to
avoid causing errors or complicating the TP project
scripts.