libcamera: ipa_module: Avoid unnecessary copy when getting signature

The `signature()` getter can just return a reference to the private vector
member variable, and let the caller make a copy if needed. Since the
return type is const qualified, this was likely the original intention.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>

include/libcamera/internal/ipa_module.h

@@ -29,7 +29,7 @@ public:
 	bool isValid() const;
 
 	const struct IPAModuleInfo &info() const;
-	const std::vector<uint8_t> signature() const;
+	const std::vector<uint8_t> &signature() const;
 	const std::string &path() const;
 
 	bool load();