fix compiling with musl

In case apt is compiled against musl-libc, then compilation fails with the following error:

cmdline/apt-internal-solver.cc:122:20: error: 'basename' was not declared in this scope; did you mean 'rename'?
|   122 |         if (strcmp(basename(argv[0]), "solver3") == 0)
|       |                    ^~~~~~~~
|       |                    rename

To fix it, include libgen header which is where this function is defined.

Once this is fixed, compilation fails once again with musl-libc, because the basename function takes char* argument instead of const char*: this is because the musl implementation cuts off the trailing slashes of the path, in case the argument is a folder path.

In our case, when passing argv[0], this is not something to worry about, since this argument is always a filename, this argument won't be actually modified, so cast it to *char.