tinyxml2: fix build in static libs configuration

tinyxml2: fix build in static libs configuration

tinyxml2 can build both static and shared libraries. By default, only
shared library is built. Shared/static builds are controlled using
separate cmake flags BUILD_SHARED_LIBS and BUILD_STATIC_LIBS.

In static libs configuration, we internally pass -DBUILD_SHARED_LIBS=OFF
cmake flag to build system which disables both shared and static builds
of library, resulting in failures while linking executable with library.

So pass -DBUILD_STATIC_LIBS=ON cmake flag in case of static libs
configuration.

fixes:
  http://autobuild.buildroot.net/results/d30/d301bcbe5db26068b35eaa94bd816ae8cf8ef2e1

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>