Add placeholder ungetc

Author: ZERICO2005

src/libc/ungetc.c

@@ -0,0 +1,9 @@
+#include "__fileioc_stdio.h"
+
+int __attribute__((weak)) ungetc(int ch, FILE *stream)
+{
+    // unimplemented
+    (void)ch;
+    (void)stream;
+    return EOF;
+}