Change dllexport controlling macro to use _WIN32

(provided by msvc compiler) instead of WIN32 (which is user configurable, and
omitted by default on some x64 builds); this fixes an issue with 64 bit windows
static library builds (#66)

Co-authored-by: Mark Sheahan <mark.sheahan@upguard.com>

include/yaml.h

@@ -28,7 +28,7 @@
 
 #if defined(__MINGW32__)
 #   define  YAML_DECLARE(type)  type
-#elif defined(WIN32)
+#elif defined(_WIN32)
 #   if defined(YAML_DECLARE_STATIC)
 #       define  YAML_DECLARE(type)  type
 #   elif defined(YAML_DECLARE_EXPORT)