fix an warning on unused local variable in fill-insn() when CAPSTONE_DIET is defined
diff --git a/cs.c b/cs.c
index 35264f4..c037de5 100644
--- a/cs.c
+++ b/cs.c
@@ -244,7 +244,9 @@
 static void fill_insn(struct cs_struct *handle, cs_insn *insn, char *buffer, MCInst *mci,
 		PostPrinter_t postprinter, const uint8_t *code)
 {
+#ifndef CAPSTONE_DIET
 	char *sp;
+#endif
 
 	if (handle->detail) {
 		// avoiding copy insn->detail