Fix powdr feature

[georgwiese]

Without this change, I get:

$ cargo run -r
error: failed to select a version for `powdr`.
    ... required by package `my-powdr-host v0.1.0 (/Users/georg/coding/powdr-template)`
versions that meet the requirements `*` (locked to 0.1.0-alpha.2) are: 0.1.0-alpha.2

the package `my-powdr-host` depends on `powdr`, with features: `simd` but `powdr` does not have these features.

failed to select a version for `powdr` which could resolve this conflict

[leonardoalt]

ah yea, I fixed this in the cargo-powdr template but not here