Clean 'shadowed declaration' warnings

index is a function in string.h, so replace index with idx all over the
source code.
diff --git a/libkmod/libkmod-index.c b/libkmod/libkmod-index.c
index 3d697d6..3fb736d 100644
--- a/libkmod/libkmod-index.c
+++ b/libkmod/libkmod-index.c
@@ -528,10 +528,10 @@
 	return new;
 }
 
-void index_file_close(struct index_file *index)
+void index_file_close(struct index_file *idx)
 {
-	fclose(index->file);
-	free(index);
+	fclose(idx->file);
+	free(idx);
 }