[patch] Fix configure test for sendfile()

[Jonathan Wakely]

One more fix for the Filesystem library: my configure test for the GNU
and Solaris version of sendfile was failing because it used NULL
without stddef.h, so we never used sendfile. That was useful, because
it meant someone found and reported a bug in the alternative
implementation that doesn't use sendfile, but it will be much faster
to use sendfile when available.

This fixes the configure test, but as it affects the build config I
won't commit it until after 6.1 is released.

Tested x86_64-linux.

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