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

Commit 494abcf (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.

.gitignore

@@ -129,7 +129,7 @@ numpy/core/src/private/npy_partition.h
 numpy/core/src/private/scalarmathmodule.h
 numpy/core/src/scalarmathmodule.c
 numpy/core/src/umath/funcs.inc
-numpy/core/src/umath/loops.c
+numpy/core/src/umath/loops.[ch]
 numpy/core/src/umath/operand_flag_tests.c
 numpy/core/src/umath/simd.inc
 numpy/core/src/umath/struct_ufunc_test.c