package/iputils: fix build without NLS

package/iputils: fix build without NLS

Set USE_GETTEXT depending on BR2_SYSTEM_ENABLE_NLS as otherwise build
will fail if gettext is not installed on host because USE_GETTEXT is
enabled by default since version 20190324 and
https://github.com/iputils/iputils/commit/2615a527209bb408b54028ef6405075cadd4541b

Fixes:
 - http://autobuild.buildroot.org/results/127823ddbd75336abc0fe0e9bd11e4f5249df87c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Acked-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>