libfdt: Forgot one function when cleaning the namespace

[Jon Loeliger]

> In commit b6d80a20fc293f3b995c3ce1a6744a5574192125, we renamed all
> libfdt functions to be prefixed with fdt_ or _fdt_ to minimise the
> chance of collisions with things from whatever package libfdt is
> embedded in, pulled into the libfdt build via that environment's
> libfdt_env.h.
> 
> Except... I missed one.  This patch applies the same treatment to
> _stringlist_contains().  While we're at it, also make it static since
> it's only used in the same file.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl