valgrind: install to staging

This is necessary for some packages depending on valgrind, such as
libdrm which will fail with an error like this one:

checking for VALGRIND... no
checking whether to enable Valgrind support... configure: error:
Valgrind support required but not present
package/pkg-generic.mk:146: recipe for target
'/br/output/build/libdrm-2.4.62/.stamp_configured' failed

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>