gallium/targets: resolve undefined reference to pipe_loader_sw_probe_dri

gallium/targets: resolve undefined reference to pipe_loader_sw_probe_dri

With the introduction of the pipe_loader_sw_probe_dri helper we
require the sw/dri winsys during linking stage despite it being
unused by any of the targets. This will cause a minor increase
in the resulting library which will be cleaned up via linker
options with upcoming patches.

v2: Link with libswdri.la only when available.

Reported-and-tested-by: Tom Stellard <thomas.stellard@amd.com> (v1)
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>