Fix build on 64 bits architectures

This upstream patch fixes a warning that, due to -Werror, is causing
build failures on 64 bits architectures.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

commit ea375582b0bfee93d66608ffc807078ffc48e961
Author: Simon Newton <nomis52@gmail.com>
Date:   Wed Oct 30 09:23:49 2013 -0700

    * Avoid a compiler warning

diff --git a/plugins/e131/e131/PreamblePacker.cpp b/plugins/e131/e131/PreamblePacker.cpp
index ef29e18..8dfcc72 100644
--- a/plugins/e131/e131/PreamblePacker.cpp
+++ b/plugins/e131/e131/PreamblePacker.cpp
@@ -77,7 +77,8 @@ const uint8_t *PreamblePacker::Pack(const PDUBlock<PDU> &pdu_block,
   if (!m_send_buffer)
     Init();
 
-  unsigned int size = MAX_DATAGRAM_SIZE - sizeof(ACN_HEADER);
+  unsigned int size = MAX_DATAGRAM_SIZE -
+      static_cast<unsigned int>(sizeof(ACN_HEADER));
   if (!pdu_block.Pack(m_send_buffer + sizeof(ACN_HEADER), &size)) {
     OLA_WARN << "Failed to pack E1.31 PDU";
     return NULL;