test/preprocess: always declare EXTERN

Unit tests never need to declare globals, only access them. In the main code
base this is handled by including "vim.h". If a file wants to declare
globals (in the case of neovim that's only main.c), it #define's EXTERN and
includes "vim.h". Otherwise, a file just includes "vim.h" (that's the
majority case). Since we want to be able to run unit tests without including
"vim.h", we predefine "EXTERN" to mean extern. That way, we don't have to
include "vim.h".

test/unit/preprocess.moon

@@ -85,6 +85,8 @@ class Gcc
    '-D "__asm(ARGS)="',
    '-D "__asm__(ARGS)="',
    '-D "__inline__="',
+   '-D "EXTERN=extern"',
+   '-D "INIT(...)="',
    '-D_GNU_SOURCE',
    '-DINCLUDE_GENERATED_DECLARATIONS'
   }