pipe-loader: fix build of dynamic pipe-drivers

pipe-loader: fix build of dynamic pipe-drivers

v2: add libxmlconfig.la to the dynamic pipe_radeonsi driver
v3: add libxmlconfig.la to targets/opencl build
v4: add EXPAT_LIBS to opencl build
    (note: for only-opencl builds, Emil's configure.ac changes
     are also needed)

Fixes: bc7f41e11d3 ("gallium: add pipe_screen_config to screen_create functions")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102014
Tested-by: Andy Furniss <adf.lists@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v1)