softpipe: Silence unused variable warning on non-LLVM builds.

Fix this GCC warning with non-LLVM builds.
sp_screen.c: In function ‘softpipe_get_shader_param’:
sp_screen.c:141:28: warning: unused variable ‘sp_screen’ [-Wunused-variable]

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/drivers/softpipe/sp_screen.c

@@ -138,7 +138,9 @@ softpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
 static int
 softpipe_get_shader_param(struct pipe_screen *screen, unsigned shader, enum pipe_shader_cap param)
 {
+#ifdef HAVE_LLVM
    struct softpipe_screen *sp_screen = softpipe_screen(screen);
+#endif
    switch(shader)
    {
    case PIPE_SHADER_FRAGMENT: