Add fallback for compute_conflict!

[odow]

If you call compute_conflict! on an unsupported solver you currently get a method error:

MathOptInterface.jl/src/MathOptInterface.jl

Lines 85 to 103 in 8075edf

	"""
	compute_conflict!(optimizer::AbstractOptimizer)
	Computes a minimal subset of constraints such that the model with the other
	constraint removed is still infeasible.
	Some solvers call a set of conflicting constraints an Irreducible Inconsistent
	Subsystem (IIS).
	See also [`ConflictStatus`](@ref) and [`ConstraintConflictStatus`](@ref).
	### Note
	If the model is modified after a call to `compute_conflict!`, the implementor
	is not obliged to purge the conflict. Any calls to the above attributes may
	return values for the original conflict without a warning. Similarly, when
	modifying the model, the conflict can be discarded.
	"""
	function compute_conflict! end