kernel: Fix compilation for non-gki kernels (#543)

kernel/file_wrapper.h

@@ -1,6 +1,10 @@
 #include <linux/file.h>
 #include <linux/fs.h>
 
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 15, 0)
+typedef unsigned int __poll_t;
+#endif
+
 struct ksu_file_wrapper {
 	struct file* orig;
 	struct file_operations ops;