ipa: libipa: histogram: Fix documentation of constructor

Commit 1dc01bc9e6 ("ipa: libipa: histogram: Add transform parameter to
constructor") added a new constructor for the Histogram class. The
implementation is inline in the header file, and the documentation was
added in the corresponding .cpp file, but it missed the \fn directive.
This causes Doxygen to complain:

src/ipa/libipa/histogram.h:31: warning: Member Histogram(Span< const uint32_t > data, Transform transform) (function) of class libcamera::ipa::Histogram is not documented.
src/ipa/libipa/histogram.cpp:59: warning: libcamera::ipa::Histogram::bins has @param documentation sections but no arguments

Fix it.

Fixes: 1dc01bc9e6 ("ipa: libipa: histogram: Add transform parameter to constructor")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>

src/ipa/libipa/histogram.cpp

@@ -50,6 +50,7 @@ Histogram::Histogram(Span<const uint32_t> data)
 }
 
 /**
+ * \fn Histogram::Histogram(Span<const uint32_t> data, Transform transform)
  * \brief Create a cumulative histogram
  * \param[in] data A (non-cumulative) histogram
  * \param[in] transform The transformation function to apply to every bin