sched_ext: Make string params of __ENUM_set() const

A small change to improve type safety/const correctness.
__COMPAT_read_enum() already has const string parameters.

It fixes a warning when using the header in C++ code:

error: ISO C++11 does not allow conversion from string literal
to 'char *' [-Werror,-Wwritable-strings]

That's because string literals have type char[N] in C and
const char[N] in C++.

Signed-off-by: Kuba Piecuch <jpiecuch@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>