BLI: fix Vector.prepend declaration

Using `&&` there was a typo. With `&&` the `prepend` method
could not be called with a const reference as argument.

source/blender/blenlib/BLI_vector.hh

@@ -637,7 +637,7 @@ class Vector {
    * Insert values at the beginning of the vector. The has to move all the other elements, so it
    * has a linear running time.
    */
-  void prepend(const T &&value)
+  void prepend(const T &value)
   {
     this->insert(0, value);
   }

source/blender/blenlib/tests/BLI_vector_test.cc

@@ -708,6 +708,17 @@ TEST(vector, Prepend)
   EXPECT_EQ_ARRAY(vec.data(), Span({7, 8, 1, 2, 3}).data(), 5);
 }
 
+TEST(vector, PrependString)
+{
+  std::string s = "test";
+  Vector<std::string> vec;
+  vec.prepend(s);
+  vec.prepend(std::move(s));
+  EXPECT_EQ(vec.size(), 2);
+  EXPECT_EQ(vec[0], "test");
+  EXPECT_EQ(vec[1], "test");
+}
+
 TEST(vector, ReverseIterator)
 {
   Vector<int> vec = {4, 5, 6, 7};