Add operator-= as random access iterators need to provide that feature.

An updated std::sort algorithm is relying on it.

Context:
llvm/llvm-project@4eddbf9

Signed-off-by: Henner Zeller <hzeller@google.com>

Author: hzeller

libs/librrgraph/src/base/rr_graph_storage.cpp

@@ -160,6 +160,11 @@ class edge_sort_iterator {
         return *this;
     }
 
+    edge_sort_iterator& operator-=(ssize_t n) {
+        swapper_.idx_ -= n;
+        return *this;
+    }
+
     edge_sort_iterator& operator++() {
         ++swapper_.idx_;
         return *this;