fix boinc build with current xcb-atom

[mjakubicek]

See the attached patch that fixes the following error when building with libxcb 1.8:

screensaver_x11.cpp: In function 'int main(int, char**)':
screensaver_x11.cpp:532:72: error: 'xcb_atom_get' was not declared in this scope
screensaver_x11.cpp:534:65: error: 'WINDOW' was not declared in this scope
screensaver_x11.cpp:550:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
screensaver_x11.cpp:555:57: error: 'WM_COMMAND' was not declared in this scope
screensaver_x11.cpp:555:69: error: 'STRING' was not declared in this scope
screensaver_x11.cpp:572:57: error: 'WM_CLASS' was not declared in this scope