package/tinyxml2: Use BUILD_SHARED_LIBS cmake standard

Backport an upstream patch to fix the static build:
c8ec6fcab4eb501cc5bd328c19dcbd4b30722d52

Fixes:
http://autobuild.buildroot.net/results/e56/e568d81030087f66f8ac4ccd4f496b550930cbae/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>