libc: Add return value to ungetc()

libs/libc/src/file.cpp

@@ -176,8 +176,12 @@ extern "C"
         {
             stream->f_bufoff--;
             stream->f_buf[stream->f_bufoff] = (char)c;
+            return c;
+        }
+        else
+        {
+            return EOF; // FIXME: Handle this case properly.
         }
-        else { NOT_IMPLEMENTED("ungetc with buffer that has not been read"); }
     }
 
     int ferror(FILE* stream)