mips64: fix typo in EXTRA_DIST

Fixes make dist error.
Typo spotted and reported by Maran Pakkirisamy <maranp@linux.vnet.ibm.com>


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13424 a5019735-40e9-0310-863c-91ae7b9d1cf9