check_msg.c: make open_tmp_file not static
check_msg.c: make open_tmp_file not static

The pack unit tests will need to have a way to create a
temporary file. Allowing them to use open_tmp_file is
simpler.

git-svn-id: caee63afe878fe2ec7dbba394a86c9654d9110ca@963 64e312b2-a51f-0410-8e61-82d0ca0eb02a

svn2github/check,svn2github/check,dashaomai/check-code,svn2github/check,dashaomai/check-code,svn2github/check,dashaomai/check-code,svn2github/check,dashaomai/check-code