seq_buf: Export seq_buf_putc()

Mark seq_buf_putc() which is part of the seq_buf API to be exported to
kernel loadable GPL modules.

Link: https://lkml.kernel.org/r/5c9a5ed97ac37dbdcd9c1e7bcbdec9ac166e79be.1698861216.git.christophe.jaillet@wanadoo.fr



Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>