libcamera: signal: Enable documentation generation from signal.h file

signal.cpp is missing a \file signal.h Doxygen directive, preventing
documentation from being generated for the Signal class. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/signal.cpp

@@ -5,6 +5,11 @@
  * signal.cpp - Signal & slot implementation
  */
 
+/**
+ * \file signal.h
+ * \brief Signal & slot implementation
+ */
+
 namespace libcamera {
 
 /**