fix new rustc warnings from beta

[m4rch3n1ng]

running cargo +beta check on this crate currently generates this warning:

warning: hiding a lifetime that's elided elsewhere is confusing
   --> src/control/property.rs:131:26
    |
131 |     pub fn convert_value(&self, value: RawValue) -> Value {
    |                          ^^^^^                      ----- the same lifetime is hidden here
    |                          |
    |                          the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
131 |     pub fn convert_value(&self, value: RawValue) -> Value<'_> {
    |                                                          ++++

warning: `drm` (lib) generated 1 warning

this pr fixes that to preempt future ci failures on the next stable rust bump in ~2w.

(note: technically cargo +beta check has slightly different wording, but the wording was changed and is being backported to beta before the next release)

[Drakulix]

thx!