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/WmInitWs.c

@@ -81,6 +81,7 @@ typedef struct
 #include <Dt/DtP.h>
 #include <Dt/Message.h>
 #include <Dt/WsmM.h>
+#include <Dt/EnvControlP.h>
 #endif /* WSM */
 
 /* Busy is also defined in the BMS  -> bms.h. This conflicts with