Fix "initializer element is not constant" error by adding #define

[Oxore]

There is such an error when using gcc on Arch Linux:

gcc -g `sdl2-config --cflags` -c src/init.c
In file included from src/init.h:1:0,
                 from src/init.c:1:
src/defs.h:25:34: error: initializer element is not constant
 static const int WINDOW_HEIGHT = PLAYFIELD_HEIGHT * ( BLOCK_SIZE + 1) + 1;
                                  ^~~~~~~~~~~~~~~~
src/defs.h:26:33: error: initializer element is not constant
 static const int WINDOW_WIDTH = PLAYFIELD_WIDTH * ( BLOCK_SIZE + 1) + 1;
                                 ^~~~~~~~~~~~~~~
In file included from src/init.h:3:0,
                 from src/init.c:1:
src/tetris.h:114:20: error: variably modified ‘playfield’ at file scope
 static Color_Block playfield[PLAYFIELD_HEIGHT * PLAYFIELD_WIDTH];
                    ^~~~~~~~~
make: *** [makefile:13: init.o] Error 1

Adding "#define" instead of "static const int" make compilation work fine.

[dashed]

@Oxore good catch! 👍