Fix compilation warning: 'GOFFICE_WITH_DECIMAL64' is not defined
The warning was:
goffice/math/goffice-math.h:37:5: warning: 'GOFFICE_WITH_DECIMAL64' is not defined, evaluates to 0 [-Wundef]
#if GOFFICE_WITH_DECIMAL64
^The warning was:
goffice/math/goffice-math.h:37:5: warning: 'GOFFICE_WITH_DECIMAL64' is not defined, evaluates to 0 [-Wundef]
#if GOFFICE_WITH_DECIMAL64
^