configure.ac: fix misdetection of libxml2

A small typo in the detection of libxml2 leads to a successful
pkg-config discovery of libxml2 to be ignored. As a consequence,
configure.ac falls back to detecting the host libxml2, which does not
work in cross-compilation situation. This commit fixes this small
typo.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>