nir: Call fflush() at the end of nir_print_shader()

We normally call with stderr which is unbuffered, so this won't affect
that, but it does let me call nir_print_shader(nir, fopen("log", "w+"))
from gdb and actually get the whole shader in my file.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

src/compiler/nir/nir_print.c

@@ -1312,6 +1312,7 @@ void
 nir_print_shader(nir_shader *shader, FILE *fp)
 {
    nir_print_shader_annotated(shader, fp, NULL);
+   fflush(fp);
 }
 
 void