Work around GCC bug with function attributes

* src/alloc.c (PNTR_ADD): Put attributes after ‘static’ and before
returned type.  Problem reported by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2018-05/msg00559.html

src/alloc.c

@@ -513,7 +513,7 @@ pointer_align (void *ptr, int alignment)
 
 #define macro_PNTR_ADD(p, i) ((p) + (i))
 
-static char * ATTRIBUTE_NO_SANITIZE_UNDEFINED ATTRIBUTE_UNUSED
+static ATTRIBUTE_NO_SANITIZE_UNDEFINED ATTRIBUTE_UNUSED char *
 PNTR_ADD (char *p, EMACS_UINT i)
 {
   return macro_PNTR_ADD (p, i);