Make all Calibrator* destructors virtual to fix warning

warning was:

main_x11.cpp: In function 'int main(int, char**)':
main_x11.cpp:42:12: warning: deleting object of abstract class type 'Calibrator' which has non-virtual destructor will cause undefined behaviour [-Wdelete-non-virtual-dtor]calibrator/Usbtouchscreen.cpp: In member function 'void CalibratorUsbtouchscreen::read_int_parameter(const char*, int&)':

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>