gupnp-av: fix static library linking with gupnp

gupnp-av: fix static library linking with gupnp

gupnp and gupnp-av defines two internal functions with the same name:
xml_util_get_element and xml_util_get_child_element_content, as a result
an application such as rygel can't be build statically.
To fix this issue, rename both functions as
xml_util_get_element2 and xml_util_get_child_element_content2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>