put WIN32_LEAN_AND_MEAN on command line, for now

pandatool/src/win-stats/Sources.pp

@@ -25,6 +25,7 @@
   #if $[DEVELOP_WINSTATS]
     #define EXTRA_CDEFS $[EXTRA_CDEFS] DEVELOP_WINSTATS
   #endif
+  #define EXTRA_CDEFS $[EXTRA_CDEFS] WIN32_LEAN_AND_MEAN
 
   #define WIN_SYS_LIBS Imm32.lib winmm.lib kernel32.lib oldnames.lib user32.lib gdi32.lib