coccinelle: ignore macro transformations in the macros themselves

For example, the following transformation:

- sizeof(s)-1
+ STRLEN(s)

would replace sizeof by STRLEN even in the STRLEN macro definition
itself, which generates following nonsensical patch:

--- src/basic/macro.h
+++ /tmp/cocci-output-8753-b50773-macro.h
@@ -182,7 +182,7 @@ static inline unsigned long ALIGN_POWER2
  *          Contrary to strlen(), this is a constant expression.
  * @x: a string literal.
  */
-#define STRLEN(x) (sizeof(""x"") - 1)
+#define STRLEN(x) (STRLEN("" x ""))

 /*
  * container_of - cast a member of a structure out to the containing structure

Let's exclude the macro itself from the transformation to avoid this