stats.wm: load mod_headers in Apache for STS

in I95196a0759b9bb36 we switched the
stats.wikimedia.org web server over to use
the global SSL settings from ssl_ciphersuite
and that also enabled Strict Transport Security
(the '365' part of the $ssl_settings line)

that results in a "Header" line in the generated
Apache config and we need to load mod_headers for that
or we are getting an error Invalid command "Header"

Change-Id: Ie9dcd4ae14fc3178c6441cb2a9a6a60cebd8dad5
Merge "stats.wm: load mod_headers in Apache for STS" into production