fastboot: Fix flashing failure when a sparse image has an avb footer.

Trying to move the AVB footer on a sparse file will corrupt the sparse
format. Rather than implement this properly, for now, have the
copy_avb_footer() function gracefully fail by skipping the operation.

Bug: 304574023
Test: fastboot flash sparse image with avb footer
Change-Id: Ia6f0711789a04897ec266ad604a3d243c7184082
diff --git a/fastboot/fastboot.h b/fastboot/fastboot.h
index 2c40890..6a49970 100644
--- a/fastboot/fastboot.h
+++ b/fastboot/fastboot.h
@@ -57,7 +57,8 @@
 };
 
 struct fastboot_buffer {
-    enum fb_buffer_type type;
+    fb_buffer_type type;
+    fb_buffer_type file_type;
     std::vector<SparsePtr> files;
     int64_t sz;
     unique_fd fd;