Correct headers location in OSGeo4W deploying

Do not use location relative to build directory, but to source directory.

This solves the problem manifesting with error like this while building on Windows with VS:
```
[ 93%] Copying OSGeo4W files
Error copying file "./include/laszip/lasunzipper.hpp" to "osgeo4w/include/laszip/".
NMAKE : fatal error U1077: 'echo' : return code '0x1'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\BIN\nmake.exe"' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\BIN\nmake.exe"' : return code '0x2'
Stop.
```