buffer: Use the correct enum type to fix a compiler warning

buffer: Use the correct enum type to fix a compiler warning

gstbuffer.c:522:58: error: implicit conversion from enumeration type 'GstBufferFlags' to
      different enumeration type 'GstBufferCopyFlags' [-Werror,-Wenum-conversion]
  if (!gst_buffer_copy_into (copy, (GstBuffer *) buffer, flags, 0, -1))
       ~~~~~~~~~~~~~~~~~~~~                              ^~~~~
gstbuffer.c:534:46: error: implicit conversion from enumeration type 'GstBufferCopyFlags' to
      different enumeration type 'GstBufferFlags' [-Werror,-Wenum-conversion]
  return gst_buffer_copy_with_flags (buffer, GST_BUFFER_COPY_ALL);
         ~~~~~~~~~~~~~~~~~~~~~~~~~~          ^~~~~~~~~~~~~~~~~~~
./gstbuffer.h:433:31: note: expanded from macro 'GST_BUFFER_COPY_ALL'
  ...((GstBufferCopyFlags)(GST_BUFFER_COPY_METADATA | GST_BUFFER_COPY_MEMORY))
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~