Update hodl.c

[Qwertylex]

I had an issue:
hodl.c: In function ‘scanhash_hodl’:
hodl.c:67:20: error: storage size of ‘ctx’ isn’t known
EVP_CIPHER_CTX ctx;
^~~
but this PR fixes it, unless I did something wrong... however it segfaults after a few seconds of benchmarking?

[bitbandi]

not good, this is just change the pointer usage.
the error "storage size of isn’t known" means, that EVP_CIPHER_CTX struct is unknown. maybe you use too old or too new openssl version.

[Qwertylex]

yes, I have openssl 1.1.0.f-2
https://www.archlinux.org/packages/core/x86_64/openssl/

...shouldn't you update your code to work with 1.1.0?
Maybe I don't know how to do that properly, sorry.