MAINT: .gitignore += generated umath/loops.h

Commit 494abcf1 (ENH: speed-up minimum, maximum and xor for bool dtype)
reworked numpy/core/src/umath/loops.h to be autogenerated at build time,
but forgot to update .gitignore.

Fix it, so that the now-autogenerated loops.h do not constantly show as
untracked in git status output.
DOC: NPY_ITEM_LISTPICKLE -> NPY_LIST_PICKLE

There is no NPY_ITEM_LISTPICKLE - it was NPY_LIST_PICKLE from the
beginning - from c3551579 (Expand usage of hasobject to be a flag-like
entity keeping track of how the data-type should be used.)

The documentation in the same rst file even uses the correct identifier
later, so it was just a typo/thinko.