pkt-line: fix declaration of set_packet_header()

When this function was changed in a97d007 (remote-curl: use
post_rpc() for protocol v2 also, 2019-02-21) from file-local to global,
the declaration was incorrectly missing the `const` qualifier.

Let's fix that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

pkt-line.h

@@ -25,7 +25,7 @@ void packet_delim(int fd);
 void packet_write_fmt(int fd, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
 void packet_buf_flush(struct strbuf *buf);
 void packet_buf_delim(struct strbuf *buf);
-void set_packet_header(char *buf, int size);
+void set_packet_header(char *buf, const int size);
 void packet_write(int fd_out, const char *buf, size_t size);
 void packet_buf_write(struct strbuf *buf, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
 void packet_buf_write_len(struct strbuf *buf, const char *data, size_t len);