DtSvc/dtwm: Fix some implicit declarations of functions by adding appropriate

Patch from Frederic Koehler <f.koehler427@gmail.com>:

These implicit definitions cause segfaults on x64 because
the implicit return type is a 32-bit signed int, rather than a pointer
type.

cde/programs/dtwm/UI.c

@@ -59,6 +59,7 @@
 #include <Xm/SeparatoG.h>
 #include <Xm/DialogS.h>
 #include <Xm/ColorObjP.h>
+#include <Xm/TextF.h>
 
 #include <X11/Xatom.h>
 #include <X11/keysymdef.h>