science/nifticlib: fix build

Add MAKE_JOBS_UNSAFE=yes to fix build:
nifti_stats.c:2:22: error: nifticdf.h: No such file or directory
nifti_stats.c: In function 'main':
nifti_stats.c:30: error: 'inam' undeclared (first use in this function)
nifti_stats.c:30: error: (Each undeclared identifier is reported only once
nifti_stats.c:30: error: for each function it appears in.)

Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20500