Update get_unchecked to void Wreturn-type-c-linkage

Otherwise:

    'cxxbridge02$std$vector$u8$get_unchecked' has C-linkage specified,
    but returns user-defined type 'const uint8_t &' (aka 'const unsigned
    char &') which is incompatible with C [-Wreturn-type-c-linkage]
diff --git a/src/cxx.cc b/src/cxx.cc
index d9be1fb..b9e4826 100644
--- a/src/cxx.cc
+++ b/src/cxx.cc
@@ -204,9 +204,9 @@
       const std::vector<CXX_TYPE> &s) noexcept {                               \
     return s.size();                                                           \
   }                                                                            \
-  const CXX_TYPE &cxxbridge02$std$vector$##RUST_TYPE##$get_unchecked(          \
+  const CXX_TYPE *cxxbridge02$std$vector$##RUST_TYPE##$get_unchecked(          \
       const std::vector<CXX_TYPE> &s, size_t pos) noexcept {                   \
-    return s[pos];                                                             \
+    return &s[pos];                                                            \
   }                                                                            \
   void cxxbridge02$std$vector$##RUST_TYPE##$push_back(                         \
       std::vector<CXX_TYPE> &s, const CXX_TYPE &item) noexcept {               \