Add missing const keyword

Signed-off-by: MTRNord <[email protected]>

src/olm/session/mod.rs

@@ -98,7 +98,7 @@ impl ChainStore {
     }
 
     #[cfg(test)]
-    pub fn len(&self) -> usize {
+    pub const fn len(&self) -> usize {
         self.inner.len()
     }