vim: include used definitions in headers

This is not an exhaustive commit, it merely ameliorates the situations a
bit. There are quite a few header files that don't include all the types
they use in their function/struct/... definitions. This throws of the
testing infrastructure (but is not such a problem for the main binary that
has the "tumbleweed of includes"-phenomenon).

src/nvim/ex_eval.h

@@ -2,6 +2,7 @@
 #define NVIM_EX_EVAL_H
 
 #include "nvim/pos.h"           // for linenr_T
+#include "nvim/ex_cmds_defs.h"  // for exarg_T
 
 /*
  * A list used for saving values of "emsg_silent".  Used by ex_try() to save the