desktop: -Werror=unused-macros

The documented feature test macro is _BSD_SOURCE, and anyway, feature
test macros must be defined before the first libc header is included, so
this one is evidently useless.

Change-Id: Ib4b2db0c6151c2f3df322992b189257f5dd0ea22
(cherry picked from commit 2e69b60cc2abb8e0a6773e101c03050c57a12ca3)