libfdt: Forgot one function when cleaning the namespace

In commit b6d80a20, 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@gibson.dropbear.id.au>