Fix to solve dllexport/import compile issues on Windows, I hope

panda/src/vision/Sources.pp

@@ -1,3 +1,4 @@
+#define BUILDING_DLL BUILDING_VISION
 #define OTHER_LIBS interrogatedb:c dconfig:c dtoolconfig:m \
                    dtoolutil:c dtoolbase:c dtool:m prc:c movies:m