sftp.h: Fix non-C90 type.

uint64_t does not exist in C90.  Use libssh2_uint64_t instead.
diff --git a/src/sftp.h b/src/sftp.h
index 332710e..2ed32ce 100644
--- a/src/sftp.h
+++ b/src/sftp.h
@@ -52,8 +52,8 @@
 
 struct sftp_pipeline_chunk {
     struct list_node node;
-    uint64_t offset; /* READ: offset at which to start reading
-    					WRITE: not used */
+    libssh2_uint64_t offset; /* READ: offset at which to start reading
+                                WRITE: not used */
     size_t len; /* WRITE: size of the data to write
                    READ: how many bytes that was asked for */
     size_t sent;