Install symlink for sys/nvpair.h in include/Makefile symlinks target

Noticed while fixing the install/sysroot situation for libnvpair and
libzfs_core- if one uses the symlinks target, libzfs_core.h is not
installed.

include/Makefile

@@ -384,6 +384,11 @@ symlinks: .PHONY .META
 		${INSTALL_SYMLINK} ${TAG_ARGS} ../../../sys/rpc/$$h \
 		    ${SDESTDIR}${INCLUDEDIR}/rpc; \
 	done
+.if ${MK_CDDL} != "no"
+	${INSTALL_SYMLINK} ${TAG_ARGS} \
+	    ../../../sys/cddl/contrib/opensolaris/uts/common/sys/nvpair.h \
+	    ${SDESTDIR}${INCLUDEDIR}/sys
+.endif
 .if ${MK_MLX5TOOL} != "no"
 	${INSTALL_SYMLINK} ${TAG_ARGS} ../../../../sys/dev/mlx5/mlx5io.h \
 	    ${SDESTDIR}${INCLUDEDIR}/dev/mlx5