Fix out-of-source compile: fix path of config.h in crypto/

[hedayat]

Don't use relative path to config.h, so that out-of source tree compiles works fine. (Main source directory is already included in -I compiler flags).

[BenWiederhake]

Damn, I was so sure that this was the "better" way. Oh well, both work for me, so I suppose this is justified. Feel free to also open a request at https://github.com/majn/tgl/ , where we keep some modifications that don't get accepted here.

CC @majn

[hedayat]

Done, although I wonder if @vysheng is opposed to this change.