libgl-xlib: Link with libunwind.

libgl-xlib: Link with libunwind.

Fix linking error.

  CXXLD    libGL.la
../../../../src/gallium/auxiliary/.libs/libgallium.a(u_debug_stack.o): In function `debug_backtrace_capture':
src/gallium/auxiliary/util/u_debug_stack.c:59: undefined reference to `_Ux86_64_getcontext'
src/gallium/auxiliary/util/u_debug_stack.c:60: undefined reference to `_ULx86_64_init_local'
src/gallium/auxiliary/util/u_debug_stack.c:62: undefined reference to `_ULx86_64_step'
src/gallium/auxiliary/util/u_debug_stack.c:71: undefined reference to `_ULx86_64_get_proc_info'
src/gallium/auxiliary/util/u_debug_stack.c:73: undefined reference to `_ULx86_64_get_proc_name'
src/gallium/auxiliary/util/u_debug_stack.c:65: undefined reference to `_ULx86_64_step'

Fixes: 70c272004f72 ("gallium/util: libunwind support")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100562
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>