Fix astro/qmapshack build with CMake 3.11

[adridg]

Summary

CMake 3.11 builds auto-moc'ed files in a slightly different order from
how CMake 3.10 did. This shows up when files don't have all their
pre-requisite includes #included in the new order.

Add the required include for a complete definition of IDrawContext, to
fix this compiler error:

src/qmapshack/qmapshack_autogen/G2GOEHWAFX/../../dem/CDemVRT.h:36:15: error: incomplete type 'IDrawContext' named in nested name specifier

void draw(IDrawContext::buffer_t& buf) override;
          ^~~~~~~~~~~~~~

src/qmapshack/canvas/CCanvas.h:31:7: note: forward declaration of 'IDrawContext'
class IDrawContext;

Test Plan

Builds on 11.1-amd64 with CMake 3.10 and CMake 3.11