[client] x11: fix failure to paste clipboard into the guest

The clipboard atoms may not exist yet and as such we must create them if
this is the case. Failure to do so results in `SEL_DATA` being zero
breaking the clipboard paste mechanics

client/displayservers/X11/atoms.c

@@ -24,7 +24,8 @@ struct X11DSAtoms x11atoms = { 0 };
 
 void X11AtomsInit(void)
 {
-  #define DEF_ATOM(x) x11atoms.x = XInternAtom(x11.display, #x, True);
+  #define DEF_ATOM(x, onlyIfExists) \
+    x11atoms.x = XInternAtom(x11.display, #x, onlyIfExists);
   DEF_ATOMS()
   #undef DEF_ATOM
 }