Declare snprintf to fix builds on macOS

For some reason on macOS, the functions `snprintf` and `vsnprintf` are
not in the X/Open 5 (ANSI C89) standard but rather in the X/Open
6 (ISO C99). A simplest solution seems to be to declaring the missing
functions before using them, which is what I did here. Another option
is to use the C99 standard with `#define _XOPEN_SOURCE 600`, which
seems to be an overkill for such a niche issue.

Quoting from the STANDARDS section in `man 3 snprintf` on macOS:

> ...the snprintf() and vsnprintf() functions conform to ISO/IEC
> 9899:1999 (“ISO C99”)...

src/devices/file.c

@@ -26,6 +26,10 @@
 #define PATH_MAX 4096
 #endif
 
+#ifdef __APPLE__
+int snprintf(char *, unsigned long, const char *, ...);
+#endif
+
 #include "../uxn.h"
 #include "file.h"