Fix clang error about incompatible assignment in simg2simg.cpp

Change-Id: Ibb5c9431e51525c083e2981979e831d7f0b8ea1d
diff --git a/libsparse/simg2simg.cpp b/libsparse/simg2simg.cpp
index a2c296e..138173b 100644
--- a/libsparse/simg2simg.cpp
+++ b/libsparse/simg2simg.cpp
@@ -72,7 +72,7 @@
     exit(-1);
   }
 
-  out_s = calloc(sizeof(struct sparse_file*), files);
+  out_s = (struct sparse_file**)calloc(sizeof(struct sparse_file*), files);
   if (!out_s) {
     fprintf(stderr, "Failed to allocate sparse file array\n");
     exit(-1);