math/xspread: drop unused libpthread-stubs

libpthread-stubs no longer provides shared library since 9cba0ef67a30:

$ make clean all WITH=X11
[...]

> xspread-3.1.1c_4 depends on shared library: libpthread-stubs.so - not found

(cherry picked from commit 04c0110c8ac4c40469614a375715188a99e74e5d)