GdextBuild::before_api(): derive expected major version from compiled API instead of hardcoding 4

Author: Bromeon

godot-codegen/src/generator/gdext_build_struct.rs

@@ -61,7 +61,7 @@ pub fn make_gdext_build_struct(header: &GodotApiVersion) -> TokenStream {
                 let mut parts = major_minor.split('.');
                 let queried_major = parts.next().unwrap().parse::<u8>().expect("invalid major version");
                 let queried_minor = parts.next().unwrap().parse::<u8>().expect("invalid minor version");
-                assert_eq!(queried_major, 4, "major version must be 4");
+                assert_eq!(queried_major, #major, "major version must be {}", #major);
 
                 let (_, minor, _) = Self::godot_runtime_version_triple();
                 minor < queried_minor