impl: fix compilation error when perf-literal is disabled

It's unclear to me why CI did not catch this. CI explicitly tests
building regex without the perf-literal feature enabled.

Author: BurntSushi

CHANGELOG.md

@@ -1,3 +1,9 @@
+1.5.1 (2021-04-30)
+==================
+This is a patch release that fixes a compilation error when the `perf-literal`
+feature is not enabled.
+
+
 1.5.0 (2021-04-30)
 ==================
 This release primarily updates to Rust 2018 (finally) and bumps the MSRV to

src/literal/mod.rs

@@ -6,7 +6,7 @@ mod imp;
 #[allow(missing_docs)]
 #[cfg(not(feature = "perf-literal"))]
 mod imp {
-    use syntax::hir::literal::Literals;
+    use regex_syntax::hir::literal::Literals;
 
     #[derive(Clone, Debug)]
     pub struct LiteralSearcher(());