Fix function pointer arguments

[tyilo]

Without this I get the following errors when compiling:

$ make
/usr/bin/gcc sslkeylog.c  -shared -o libsslkeylog.so -fPIC -ldl 
sslkeylog.c: In function ‘SSL_new’:
sslkeylog.c:119:9: error: too many arguments to function ‘set_keylog_cb’; expected 0, have 2
  119 |         set_keylog_cb(ctx, keylog_callback);
      |         ^~~~~~~~~~~~~ ~~~
sslkeylog.c:121:12: error: too many arguments to function ‘func’; expected 0, have 1
  121 |     return func(ctx);
      |            ^~~~ ~~~
make: *** [Makefile:2: libsslkeylog.so] Error 1