Missing include in unicode_support.c results in compiler warning

## Repro
Compile the library using MSVC 2019, see warning:

```
libopusenc\src\unicode_support.c(44,56): warning C4312: 'type cast': conversion from 'int' to 'wchar_t *' of greater size
```

## Solution
The malloc function is used without including its definition. Including "stdlib.h" results in no warning.