Warnings about deprecated gssapi.h

[Пётр Воропаев]

Hello.

Currecntly using FreeBSD 10.0, and I was kinda annoyed by multiple lines of:

In file included from ../../net/http/http_auth_gssapi_posix.h:20:
/usr/include/gssapi.h:3:2: warning: "this file includes <gssapi.h> which is
deprecated, use <gssapi/gssapi.h> instead" [-W#warnings]
#warning "this file includes <gssapi.h> which is deprecated, use
<gssapi/gssapi.h> instead"

in my build log. And I feared that after gssapi.h ends up finally removed,
chromium would fail building on FreeBSD.

After some limited digging into sourcecode it was discovered that Chromium
supports case "And what if target OS is FreeBSD" by having a OS_FREEBSD
defined in build/build_config.h

Candidate to future-proofing patch included.