at: Remove mktime signature from posixtm.c

This is in conflict with signature from system time.h

extern time_t mktime (struct tm *__tp) __THROW;

GCC-15 with C23 on, this is treated at error

 posixtm.c:45:8: error: conflicting types for 'mktime'; have 'time_t(void)' {aka 'long int(void)'}
|    45 | time_t mktime ();
|       |        ^~~~~~
|

config.h is local include file to use quotes to include it

(From OE-Core rev: afc888d87fbf268c43bbcc81aa378eb9d58397a8)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-extended/at/at/posixtm.c

@@ -20,7 +20,7 @@
 /* Yacc-based version written by Jim Kingdon and David MacKenzie.
    Rewritten by Jim Meyering.  */
 
-#include <config.h>
+#include "config.h"
 
 #include "posixtm.h"
 
@@ -42,8 +42,6 @@
    of `digit' even when the host does not conform to POSIX.  */
 #define ISDIGIT(c) ((unsigned int) (c) - '0' <= 9)
 
-time_t mktime ();
-
 /*
   POSIX requires: