prevent precompilation warning for upcoming versions of Julia

Fixes this precompilation warning:

```julia-repl
julia> using LinearSolve
Info Given LinearSolve was explicitly requested, output will be shown live
WARNING: Detected access to binding `LinearSolve.defaultalg_symbol` in a world prior to its definition world.
  Julia 1.12 has introduced more strict world age semantics for global bindings.
  !!! This code may malfunction under Revise.
  !!! This code will error in future versions of Julia.
Hint: Add an appropriate `invokelatest` around the access to this binding.
To make this warning an error, and hence obtain a stack trace, use `julia --depwarn=error`.
Precompiling LinearSolve finished.
  77 dependencies successfully precompiled in 85 seconds. 32 already precompiled.
  1 dependency had output during precompilation:
┌ LinearSolve
│  [Output was shown above]
└
```

Author: nsajko

src/LinearSolve.jl

@@ -140,6 +140,8 @@ end
 
 const BLASELTYPES = Union{Float32, Float64, ComplexF32, ComplexF64}
 
+function defaultalg_symbol end
+
 include("generic_lufact.jl")
 include("common.jl")
 include("extension_algs.jl")