Fix YARA compiler builder pattern usage

The add_rules_str() method consumes self and returns a new Compiler,
following a builder pattern. Updated to reassign the returned compiler
instead of treating it as a mutable method.

This fixes the compilation error when building with yara-scanning feature.

ghost-core/src/yara_engine.rs

@@ -171,10 +171,13 @@ impl DynamicYaraEngine {
                         .and_then(|s| s.to_str())
                         .unwrap_or("default");
 
-                    if let Err(e) = compiler.add_rules_str(&content) {
+                    compiler = match compiler.add_rules_str(&content) {
+                        Ok(c) => c,
+                        Err(e) => {
                             log::error!("Failed to compile {}: {}", rule_file.display(), e);
                             continue;
                         }
+                    };
 
                     log::info!("Compiled YARA rule: {}", rule_file.display());