libsparse: fix windows image writing

Fix write_fd_chunk on windows.  Uses malloc and read instead of mmap.

Change-Id: I75f10db2e04f19e7f3a6ff46b6978d143cb5254e
diff --git a/libsparse/output_file.h b/libsparse/output_file.h
index d23abf3..b86528b 100644
--- a/libsparse/output_file.h
+++ b/libsparse/output_file.h
@@ -36,4 +36,6 @@
 int write_skip_chunk(struct output_file *out, int64_t len);
 void close_output_file(struct output_file *out);
 
+int read_all(int fd, void *buf, size_t len);
+
 #endif