qt: Webkit Module needs a toolchain with NPTL

Qt Webkit Module uses pthread_getattr_np() which is implemented in
toolchains with NPTL. Otherwise the compilation will fail showing this
error message:

wtf/StackBounds.cpp: In member function ‘void
WTF::StackBounds::initialize()’:
wtf/StackBounds.cpp:172:38: error: ‘pthread_getattr_np’ was not declared
in this scope

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>