The buildbots fail to build socat due to the following error:

    nestlex.c:14:7: error: unknown type name 'ptrdiff_t'

It appears that certain source files do not include all required headers,
depending on the configure options passed to socat.

Work around the error by passing `-include stddef.h` via `TARGET_CFLAGS` to
forcibly inject this header file into all compilation units.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>