libglib2: fix build when toolchain has no thread support

Fixes the build failure visible at
http://autobuild.buildroot.net/results/3d3/3d3f4f0044a4b58ce3d0a3449a5f012d953d59bb



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>