meta: add msvc support

since bluelib is already cross-platform, this is mostly just adding
__declspec(dllexport) to the library functions.

common/include/ivy/line-source.h

@@ -9,7 +9,7 @@ struct ivy_line_source {
 		struct ivy_line_source *, char *, size_t, size_t *, const char *);
 };
 
-extern enum ivy_status ivy_line_source_readline(
+IVY_API enum ivy_status ivy_line_source_readline(
 	struct ivy_line_source *src, char *buf, size_t count, size_t *nr_read,
 	const char *prompt);