remove unused function mkfullpath from windows build

This is a quick fix, if you have a better one, please reject.

Explanation: Building netvirt-agent on windows complains about the usage of `mkdir`: 
```text
nvagent/src/config.c:50:3: error: too many arguments to function ‘mkdir’
```
For i686-w64-mingw32, `mkdir` is declared as:
```c
int __cdecl mkdir (const char *) __MINGW_ATTRIB_DEPRECATED_MSVC2005;
```
Plus, `mkfullpath` is only used for Linux/BSD