Fix compilation warnings/errors in e131/PDU.cpp
Fix compilation warnings/errors in e131/PDU.cpp

The errors I am getting with gcc 5.1.0 are:

../plugins/e131/e131/PDU.cpp: In member function ‘virtual void ola::plugin::e131::PDU::Write(ola::io::OutputStream*) const’:
../plugins/e131/e131/PDU.cpp:124:55: error: conversion to ‘uint16_t {aka short unsigned int}’ from ‘int’ may alter its value [-Werror=conversion]
	 ((VFLAG_MASK | HFLAG_MASK | DFLAG_MASK) << 8u));
						       ^
../plugins/e131/e131/PDU.cpp:128:71: error: conversion to ‘uint8_t {aka unsigned char}’ from ‘int’ may alter its value [-Werror=conversion]
			  static_cast<uint8_t>((size & 0x0f0000) >> 16));
								       ^
../plugins/e131/e131/PDU.cpp: In static member function ‘static void ola::plugin::e131::PDU::PrependFlagsAndLength(ola::io::OutputBufferInterface*, unsigned int, uint8_t)’:
../plugins/e131/e131/PDU.cpp:170:43: error: conversion to ‘uint16_t {aka short unsigned int}’ from ‘int’ may alter its value [-Werror=conversion]
	 static_cast<uint16_t>(flags << 8u));
					   ^
../plugins/e131/e131/PDU.cpp:177:25: error: conversion to ‘uint8_t {aka unsigned char}’ from ‘int’ may alter its value [-Werror=conversion]
     flags_and_length[0] = (flags |
			 ^

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