Add -lpthread to liblxrt LDFLAGS

dpkg found `liblxrt.so` isn't being linked with `libpthread`:

     dpkg-shlibdeps: warning: symbol __pthread_register_cancel used by debian/librtai1/usr/lib/liblxrt.so.1.0.0 found in none of the libraries
     dpkg-shlibdeps: warning: symbol pthread_create used by debian/librtai1/usr/lib/liblxrt.so.1.0.0 found in none of the libraries
     dpkg-shlibdeps: warning: symbol pthread_testcancel used by debian/librtai1/usr/lib/liblxrt.so.1.0.0 found in none of the libraries
     dpkg-shlibdeps: warning: symbol pthread_attr_setstacksize used by debian/librtai1/usr/lib/liblxrt.so.1.0.0 found in none of the libraries
     dpkg-shlibdeps: warning: symbol __pthread_unregister_cancel used by debian/librtai1/usr/lib/liblxrt.so.1.0.0 found in none of the libraries
     dpkg-shlibdeps: warning: symbol pthread_join used by debian/librtai1/usr/lib/liblxrt.so.1.0.0 found in none of the libraries
     dpkg-shlibdeps: warning: symbol pthread_kill used by debian/librtai1/usr/lib/liblxrt.so.1.0.0 found in none of the libraries