Marked compiler literal unsigned.

When compiling a protobuf with gcc 3.3.2 for powerpc, I ran into the
following warning message:

INFO: From Compiling my_proto.pb.cc powerpc-603e-linux-gcc:
bazel-out/local_linux-dbg/genfiles/my_proto.pb.cc: In member
   function `virtual void MyProto::Clear()':
   bazel-out/local_linux-dbg/genfiles/my_proto.pb.cc:223: warning: this
      decimal constant is unsigned only in ISO C90

The line in the proto file that was triggering it was:

  if (_has_bits_[24 / 32] & 4278190080) {
    ZR_(field1_, field2_);
  }

_has_bits_ is a uint32.  The constant mask should therefore be
unsigned.  This change updates the constant to be generated as
unsigned.