Fix warning on all c++ files

Since vlc_atomic_float is only used for C files, another option would be
to disable this block when building in C++ mode.
Related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64658

include/vlc_atomic.h

@@ -115,7 +115,12 @@ static inline void vlc_atomic_init_float(vlc_atomic_float *var, float f)
 {
     union { float f; uint32_t i; } u;
     u.f = f;
+#if defined(__cplusplus) && defined(HAVE_CXX11)
+    vlc_atomic_float a(u.i)
+    *var = a;
+#else
     atomic_init(var, u.i);
+#endif
 }
 
 /** Helper to retrieve a single precision from an atom. */