[librsvg/rustify-rsvg-convert] Cargo: be explicit about the version of rust required

[Paolo Borelli <pborelli src gnome org>]

commit 5e44eda5cc930aa7abc093de13c3214ea452d34c
Author: Paolo Borelli <pborelli gnome org>
Date:   Sun Jan 17 10:40:12 2021 +0100

    Cargo: be explicit about the version of rust required

 Cargo.toml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/Cargo.toml b/Cargo.toml
index 84152e52..14072f3d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,6 +4,7 @@ version = "2.51.0"
 authors = ["Federico Mena Quintero <federico gnome org>"]
 build = "build.rs"
 edition = "2018"
+rust = 1.42
 
 [lib]
 name = "librsvg"