make it build for mingw targets

https://bugs.gentoo.org/393573

--- a/bzlib.h
+++ b/bzlib.h
@@ -81,6 +81,9 @@ typedef
       /* windows.h define small to char */
 #      undef small
 #   endif
+#   ifndef WINAPI
+#   define WINAPI
+#   endif
 #   ifdef BZ_EXPORT
 #   define BZ_API(func) WINAPI func
 #   define BZ_EXTERN extern