Make Cosmopolitan ANSI C89 compatible

You can now use cosmopolitan.h with an ANSI C89 compiler like MSVC. The
Cosmopolitan codebase itself won't support being compiled that way. But
you can build objects that link against Cosmopolitan using any compiler
and you can furthermore use tools like IntelliSense that can't even GNU

See also #40

libc/x/xvasprintf.c

@@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/log/log.h"
+#include "libc/mem/fmt.h"
 #include "libc/mem/mem.h"
 #include "libc/x/x.h"