BLD: fix msvc compiler errors

```
_test_multivariate.c
scipy\integrate\tests\_test_multivariate.c(24) : error C2057: expected constant expression
scipy\integrate\tests\_test_multivariate.c(24) : error C2466: cannot allocate an array of constant size 0
scipy\integrate\tests\_test_multivariate.c(30) : error C2057: expected constant expression
scipy\integrate\tests\_test_multivariate.c(30) : error C2466: cannot allocate an array of constant size 0
scipy\integrate\tests\_test_multivariate.c(36) : error C2057: expected constant expression
scipy\integrate\tests\_test_multivariate.c(36) : error C2466: cannot allocate an array of constant size 0
```