Add an ifdef allowing to rename Solvable dependency members

Add an ifdef allowing to rename Solvable dependency members

In C++20 "requires" is a new keyword, causing a conflict with the
Solvable::requires attribute. From the outside this can only be fixed
with an ugly preprocessor redefinition of the symbol around each libsolv
header include.

This commit adds an #ifdef allowing to rename the dep attributes in case
C++20 or higher is used, which can be easily defined in one place in the
libolv user's build system.