libstrophe: fix detection of libxml2

This commit adds a patch to libstrophe to fix a typo that prevented a
proper detection of libxml2. This patch has been submitted upstream,
see https://github.com/metajack/libstrophe/pull/46.

Fixes:

 http://autobuild.buildroot.org/results/499/499ffd50ec58b54343d6606c0f1bf7cd04f14e1c/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>