libroxml: fix pthread linking issue

Fixes http://autobuild.buildroot.net/results/d18/d188bcf5c374273ffe09589f7db5a40e4c5d7bc9/



Thread handling can get disabled with --disable-xml-thread-safe, but that's
a change for next.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>