C++ Used In IGA Headers

When using `kv.h` within a C program, the following errors occur:

```
include/iga/iga_types_swsb.hpp:14:1: error: unknown type name 'namespace'
...
include/iga/kv.h:99:24: error: unknown type name 'kv_t'
...
```

These errors are due to `kv.h` including a C++ header file.