Small fix in platform.h, just for windows (WIN32)

Not sure how this has lasted so long, but I guess there are not too many
users wanting to specifically use the static tidy library.

Now it is possible to define TIDY_STATIC and the TIDY_EXPORT macro will
just be 'external', suitable for linking with the static library, instead
of adding the _imp_ MSVC __declspec( dllimport )...

This fix does not effect other than the WIN32 platform build.

include/platform.h

@@ -495,7 +495,7 @@ extern "C" {
 
 #if defined(_WIN32)
 
-#if (defined(_USRDLL) || defined(_WINDLL)) || defined(BUILD_SHARED_LIB) && !defined(TIDY_EXPORT)
+#if (defined(_USRDLL) || defined(_WINDLL) || defined(BUILD_SHARED_LIB)) && !defined(TIDY_EXPORT) && !defined(TIDY_STATIC)
 #ifdef BUILDING_SHARED_LIB
 #define TIDY_EXPORT __declspec( dllexport ) 
 #else