xfsprogs: Define PLATFORM to linux

PLATFORM is an environment variable used by xfsprogs' configure script
to determine the platform for which the applications are being built. If
we set some incorrect/unsupported value through e.g: export, this will
be picked up by xfsprogs' configure script and used as-is and assigned
to PKG_PLATFORM, which will lead to build failures.

If PLATFORM was empty/unset, then uname on the host building xfsprogs
gets used to determine the build platform, which again could be
incorrect if we e.g: built xfsprogs on a Darwin system.

Since we are obviously building for Linux, let's just make sure we
define it that way which solves both issues.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>