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/buffer.h

@@ -2,6 +2,7 @@
 #define NVIM_BUFFER_H
 
 #include "nvim/pos.h"  // for linenr_T
+#include "nvim/ex_cmds_defs.h"  // for exarg_T
 
 /* Values for buflist_getfile() */
 #define GETF_SETMARK    0x01    /* set pcmark before jumping */