Patch for build error on Ubuntu LTS

Greetings abiword devs,

abiword 3.0.2 (latest release) fails to build on Ubuntu 16.04.2 LTS
using g++ 5.4.0 with these fatal compile-time errors:

gr_UnixCairoGraphics.cpp:183:37: error: ‘nullptr’ was not declared in this scope
  m_styleBg = XAP_GtkStyle_get_style(nullptr, "GtkButton"); // "button"
gr_UnixCairoGraphics.cpp:630:30: error: ‘nullptr’ was not declared in this scope
   GtkStyleContext *context = nullptr;

These errors are trivial to fix by replacing the undeclared 'nullptr'
identifier with the literal '0', after which abiword builds and runs
properly. A patch that does this is attached.

Be well,
    -- Jacob