Add Windows support for shared libraries

[traversaro]

The package compiles a library, but does not expose any symbol on Windows, so if the CMake project is compiled with -DBUILD_SHARED_LIBS:BOOL=ON on Windows, no library is actually generated.

On Linux and macOS, everything compiles fine with -DBUILD_SHARED_LIBS:BOOL=ON as by default all the symbols are visible. We can achieve exactly the same behavior in Windows by setting to ON the CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS CMake variable, so this PR sets the CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS variable to ON, to ensure that the compilation with -DBUILD_SHARED_LIBS:BOOL=ON works fine on Windows.