ltp-testsuite: add numactl as optional dependency

Make the detection of libnuma in the configure script consistent when
the numactl package is enabled.

ltp-testsuite does not currently take explicit enable/disable for
libnuma, so none are used. The next ltp-testsuite version will add these
options.

Cc: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>