Fix warning about "comparison between signed and unsigned" in example and make...

Fix warning about "comparison between signed and unsigned" in example and make it fully C89 compatible.

docs/examples/memoryviews/C_func_file.c

@@ -2,7 +2,8 @@
 
 void multiply_by_10_in_C(double arr[], unsigned int n)
 {
-    for (int i = 0; i < n; i++) {
+    unsigned int i;
+    for (i = 0; i < n; i++) {
         arr[i] *= 10;
     }
 }