st/nir: use asprintf() wrapper to fix MSVC issues

Fixes: 856e84083e ("mesa/st: add sampler uniforms")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/state_tracker/st_nir_lower_tex_src_plane.c

@@ -29,6 +29,7 @@
  * This should run after nir_lower_samplers.
  */
 
+#include "util/u_string.h"
 #include "compiler/nir/nir.h"
 #include "st_nir.h"