MAINT: Define NPY_NO_DEPRECATED_API in struct_ufunc_test.c.src

Removes a compile warning and makes the function use proper macro
replacements. Also fix a pointer type mismatch warning.
Merge pull request #3933 from charris/add-missing-no-deprecated-api

MAINT: Define NPY_NO_DEPRECATED_API in struct_ufunc_test.c.src