linprocfs: Fix formatting of Uid and Gid lines

The separator here should be tabs, not spaces. This fixes a warning
from chromium-browser on Bionic:

[1022/162248.137612:ERROR:process_info_linux.cc(107)] format error: unrecognized Uid format

Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32612