Added xp_dl.c to the single-threaded xpdev msvc6 project file too.

BTW, there is this interesting msvc6 warning:
xp_dl.h(17) : warning C4229: anachronism used : modifiers on data are ignored

src/xpdev/xpdev.dsp

@@ -126,6 +126,10 @@ SOURCE=.\str_list.c
 # End Source File
 # Begin Source File
 
+SOURCE=.\xp_dl.c
+# End Source File
+# Begin Source File
+
 SOURCE=.\xpdatetime.c
 # End Source File
 # Begin Source File