Update HSL_MC68 header

[jfowkes]

We have found a minor bug in the HSL_MC68 C header that we have been shipping to date, that is the control.min_l_workspace parameter had the wrong type in the C header. This has now been corrected so that it has integer type as is the case in the rest of the C interface and the Fortran code. Please see the HSL_MC68 changelog:
https://www.hsl.rl.ac.uk/catalogue/hsl_mc68.html

I can only assume that this optional parameter was not used as we would expect the mismatch in structs on the C and Fortran side to cause mangling of members in the C control struct (this is a major weakness of Fortran ISO C interfacing).

[svigerske]

Thanks. Indeed, it seems that only the f_array_in and f_array_out members are accessed by the HSL_MA77 and HSL_MA86 interfaces.