tools/nolibc: use __inline__ syntax

When building in strict C89 mode the "inline" keyword is unknown.
While "__inline__" is non-standard it is used by the kernel headers
themselves.
So the used compilers would have to support it or the users shim it with
a #define.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>