Move PMTU header block to top of file

Move PMTU header block to top of file

1206e7538be86f073b21cd289266286b60a95d0a added linux/errqueue.h to
rx_user.c, but added the include in the middle of a function - which
means that the new structure is out of scope for the rest of the file,
which breaks the build on Linux.

Put the header include at the start with all of its other friends.

Cc: Alf Wachsmann <alfw@slac.stanford.edu>
Change-Id: Id932ff63b59e6cd62eda3df4f01e4eef354736ca
Reviewed-on: http://gerrit.openafs.org/748
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>