samples: shields: npm6001_ek: fix build

Commit ff3aaa6ef30bf08f2a7490b3608f10b64c1821c3 moved some getopt
declarations to unistd.h. Update sample to include the header, so that
we can access optarg.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
1 file changed