MAINT: fix source files that were incorrectly listed in .gitignore

To find such files:
```
import os

with open('../numpy/.gitignore') as f:
    ignored = f.readlines()
    ignored = [s.split('\n')[0] for s in ignored if s.startswith('numpy')]

for fname in ignored:
    if os.path.exists(fname):
        print(fname)
```

.gitignore

@@ -132,9 +132,6 @@ numpy/core/include/numpy/config.h
 numpy/core/include/numpy/multiarray_api.txt
 numpy/core/include/numpy/ufunc_api.txt
 numpy/core/lib/
-numpy/core/src/common/npy_binsearch.h
-numpy/core/src/common/npy_cpu_features.c
-numpy/core/src/common/npy_partition.h
 numpy/core/src/common/npy_sort.h
 numpy/core/src/common/templ_common.h
 numpy/core/src/multiarray/_multiarray_tests.c
@@ -160,8 +157,6 @@ numpy/core/src/npysort/sort.c
 numpy/core/src/private/npy_binsearch.h
 numpy/core/src/private/npy_partition.h
 numpy/core/src/private/templ_common.h
-numpy/core/src/umath/_rational_tests.c
-numpy/core/src/umath/_struct_ufunc_tests.c
 numpy/core/src/umath/_umath_tests.c
 numpy/core/src/umath/scalarmath.c
 numpy/core/src/umath/funcs.inc