mesa/st: Use 'struct nir_shader' instead of 'nir_shader'.

mesa/st: Use 'struct nir_shader' instead of 'nir_shader'.

Fix this build error with GCC 4.4.

  CC     state_tracker/st_nir_lower_builtin.lo
In file included from state_tracker/st_nir_lower_builtin.c:61:
state_tracker/st_nir.h:34: error: redefinition of typedef ‘nir_shader’
../../src/compiler/nir/nir.h:1830: note: previous declaration of ‘nir_shader’ was here

Suggested-by: Rob Clark <robdclark@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96235
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Rob Clark <robdclark@gmail.com>