Can't compile

On ubuntu 18.04, kernel 4.15.0, OpenSSL 1.1.1:

```
~/svn/ktls-example {master}$ make
make -C lib
make[1]: Entering directory '/home/giampaolo/svn/ktls-example/lib'
cc -Wall -Werror -g -O2 -I../include -c -o ktls.o ktls.c
ktls.c: In function ‘setup_ktls’:
ktls.c:72:34: error: dereferencing pointer to incomplete type ‘SSL {aka struct ssl_st}’
EVP_CIPHER_CTX * write_ctx = ssl->enc_write_ctx;
^~
ktls.c:73:59: error: dereferencing pointer to incomplete type ‘EVP_CIPHER_CTX {aka struct evp_cipher_ctx_st}’
EVP_AES_GCM_CTX* gcm_write = (EVP_AES_GCM_CTX*)(write_ctx->cipher_data);
^~
Makefile:15: recipe for target 'ktls.o' failed
make[1]: *** [ktls.o] Error 1
make[1]: Leaving directory '/h4.15.0ome/giampaolo/svn/ktls-example/lib'
Makefile:29: recipe for target 'lib' failed
make: *** [lib] Error 2
```

Start by running make on Ubuntu 18.04 with kernel 4.15.0 and OpenSSL 1.1.1, then inspect lib/ktls.c around setup_ktls and the related Makefile rules. Determine why the OpenSSL types are incomplete in this environment; done means the project compiles successfully with the reported toolchain.

Written by the indexing model from the issue text.