glsl: Link glsl_test with pthreads library.
glsl: Link glsl_test with pthreads library.

Otherwise pthread_mutex_lock will be an undefined reference
on OpenBSD.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88219
Signed-off-by: Jonathan Gray <666d82badf37dc240bacece830d7f52411b6fee3@jsg.id.au>
Reviewed-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Cc: "10.4 10.3" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler