Makefile.bsd: remove directory that no longer exists

This was removed in a reorganization of directories preparing for the
merge of FreeBSD support, 006e9a408824 by mmacy. While llvm is perfectly
happy with the nonexistent -I directory, the gcc6 and gcc9 we can elect
to use as cross-toolchains both trip over it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Kyle Evans <kevans@FreeBSD.org>
Closes #11077