Specify private libraries required for static linking.

Otherwise there will be lots of error like the following
```
...
/home/.../libde265.a(libde265_la-de265.o): In function `de265_new_decoder':
/home/.../libde265/libde265/de265.cc:181: undefined reference to `__gxx_personality_sj0'
/home/.../libde265/libde265/de265.cc:187: undefined reference to `operator new(unsigned int)'
/home/.../libde265/libde265/de265.cc:187: undefined reference to `operator delete(void*)'
...
```