fix(vfs): Include lib/printf.h for snprintf declaration across ARM32 and core kernel

- Add #include <lib/printf.h> in fs/vfs.c, include/lib/string.h, and include/kernel/printk.h
- Ensure snprintf is always declared during freestanding compilation

include/kernel/printk.h

@@ -4,6 +4,7 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdarg.h>
+#include <lib/printf.h>
 
 // Linux-compatible log level prefixes
 #define KERN_EMERG   "\0010"  /* system is unusable */