Fix compilation with -Werror=format-security

The Debian package of spice-xpi failed to build on powerpc and sparc in
Debian, failing with this error message:

  make[6]: Entering directory `/«PKGBUILDDIR»/SpiceXPI/src/plugin'
    CXX    libnsISpicec_la-controller.lo
  controller.cpp: In destructor 'SpiceController::~SpiceController()':
  controller.cpp:73:5: error: format not a string literal and no format
    arguments [-Werror=format-security]
  cc1plus: some warnings being treated as errors

The error is in a call to g_debug(G_STRFUNC). As this is a constant string,
we can just replace the call with g_debug("%s", G_STRFUNC).