cocci: Help spatch operate on VMODs

Successfully tested with a dummy semantic patch:

    @@
    idexpression ctx, caller;
    @@

    -CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
    +AN(ctx);
     CHECK_OBJ_NOTNULL(caller, DEBUG_CALLER_MAGIC);
    -AN(caller->sub);
    +assert(caller->sub);

I decided to cram all definitions in the existing vdef.h file to keep
our relatively simple setup. We can add more macros as we learn what
coccinelle trips over.

Refs f5b0b201

tools/coccinelle/vdef.h

+/* vdef.h */
 #define v_printflike_(f,a)
 #define v_deprecated_
 #define v_dont_optimize
 #define v_matchproto_(xxx)
 #define v_statevariable_(varname)
 #define v_unused_
+
+/* vrt.h */
+#define VRT_CTX	const struct vrt_ctx *ctx
+
+/* vcc_if.h */
+#define VPFX(a)	vmod_##a