CMake: Fix pandatool/p3txafile build error on Windows

Need to define BUILDING_MISC for p3txafile so that it can
be properly exported.

pandatool/src/egg-palettize/CMakeLists.txt

@@ -3,5 +3,6 @@ target_link_libraries(egg-palettize p3palettizer p3eggbase p3progbase)
 install(TARGETS egg-palettize DESTINATION bin)
 
 add_library(p3txafile txaFileFilter.cxx txaFileFilter.h txaFileFilter.I)
+set_target_properties(p3txafile PROPERTIES DEFINE_SYMBOL BUILDING_MISC)
 target_link_libraries(p3txafile p3palettizer)
 install(TARGETS p3txafile DESTINATION lib RUNTIME DESTINATION bin)