test: libtool --mode=execute valgrind

otherwise valgrind will not actually run against the binary.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2504 from dachary/wip-erasure-code-valgrind

test: libtool --mode=execute valgrind

Reviewed-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@redhat.com>