Makefile.bsd: remove directory that no longer exists

This was removed in a reorganization of directories preparing for the
merge of FreeBSD support, 006e9a4 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 <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Kyle Evans <[email protected]>
Closes #11077

Author: kevans91

module/Makefile.bsd

@@ -24,7 +24,6 @@ KMOD=	openzfs
 
 CFLAGS+= -I${.OBJDIR:H}/include
 CFLAGS+= -I${INCDIR}
-CFLAGS+= -I${INCDIR}/spl
 CFLAGS+= -I${INCDIR}/os/freebsd
 CFLAGS+= -I${INCDIR}/os/freebsd/spl
 CFLAGS+= -I${INCDIR}/os/freebsd/zfs