Explicitely define RawDecodingSettings copy constructor

and default the assignment operator.

Fixes gcc warning

libkdcraw/src/kdcraw.cpp: In static member function ‘static bool KDcrawIface::KDcraw::loadFullImage(QImage&, const QString&, const KDcrawIface::RawDecodingSettings&)’:
libkdcraw/src/kdcraw.cpp:242:31: warning: implicitly-declared ‘KDcrawIface::RawDecodingSettings::RawDecodingSettings(const KDcrawIface::RawDecodingSettings&)’ is deprecated [-Wdeprecated-copy]
  242 |     RawDecodingSettings prm = settings;
      |                               ^~~~~~~~
In file included from libkdcraw/src/kdcraw.h:27,
                 from libkdcraw/src/kdcraw.cpp:9:
libkdcraw/src/rawdecodingsettings.h:151:26: note: because ‘KDcrawIface::RawDecodingSettings’ has user-provided ‘KDcrawIface::RawDecodingSettings& KDcrawIface::RawDecodingSettings::operator=(const KDcrawIface::RawDecodingSettings&)’
  151 |     RawDecodingSettings& operator=(const RawDecodingSettings& prm);
      |                          ^~~~~~~~