Fix VPATH builds for user config

cmd/zpool and lib/libzutil Makefile's use -I., which won't work with a
VPATH build. Replace it with -I$(srcdir) instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10379
Closes #10421