ring_buffer.h: fix GCC warning about unused function

ring_buffer.h: fix GCC warning about unused function

With register_ring_buffers() being declared as "static" in header file,
all translation units, which include that header, got a copy of that
function.
This causes GCC warning

  warning: "register_ring_buffers" defined but not used [-Wunused-function]

when compiling C files which include header, but don't use function.

Add "inline" keyword to silence this warning.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20211124160347.1245-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23260.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 71371f04afa6eba3ea02a67590a70e018cf203e5)