Use array indexing with considering clang's feeling

```
In file included from sprintf.c:1256:
./vsnprintf.c:833:8: warning: adding 'int' to a string does not append to the string [-Wstring-plus-int]
                            IS_PRI_EXTRA_MARK(fmt)) {
                            ^~~~~~~~~~~~~~~~~~~~~~
./vsnprintf.c:826:34: note: expanded from macro 'IS_PRI_EXTRA_MARK'
           strncmp((s)+1, PRI_EXTRA_MARK+1, \
                          ~~~~~~~~~~~~~~^~
./vsnprintf.c:833:8: note: use array indexing to silence this warning
./vsnprintf.c:826:34: note: expanded from macro 'IS_PRI_EXTRA_MARK'
           strncmp((s)+1, PRI_EXTRA_MARK+1, \
                                        ^
1 warning generated.
```

vsnprintf.c

@@ -823,7 +823,7 @@ reswitch:	switch (ch) {
 	(PRI_EXTRA_MARK_LEN < 1 || \
 	 (*(s) == PRI_EXTRA_MARK[0] && \
 	  (PRI_EXTRA_MARK_LEN == 1 || \
-	   strncmp((s)+1, PRI_EXTRA_MARK+1, \
+	   strncmp((s)+1, &PRI_EXTRA_MARK[1], \
 		   PRI_EXTRA_MARK_LEN-1) == 0)))
 #else
 # define PRI_EXTRA_MARK_LEN 0