buffer: fix gcc 4.2 build breakage

gcc 4.2 on OS X gets confused about the call to node::Buffer::Data().
Fully qualify the function name to help it along.

Fixes the following build error:

    ../../deps/v8/include/v8.h: In function ‘char*
    node::Buffer::Data(v8::Handle<v8::Value>)’:
    ../../deps/v8/include/v8.h:900: error: ‘class v8::Data’
    is not a function,
    ../../src/node_buffer.h:38: error:
    conflict with ‘char* node::Buffer::Data(v8::Handle<v8::Object>)’
    ../../src/node_buffer.cc:94: error:
    in call to ‘Data’