crt: testcases: Use stdlib.h for the declaration of _sys_errlist

This fixes linking this test with UCRT.

Signed-off-by: Martin Storsjö <martin@martin.st>

mingw-w64-crt/testcases/tstmain_sys_xxx.c

diff --git a/mingw-w64-crt/testcases/tstmain_sys_xxx.c b/mingw-w64-crt/testcases/tstmain_sys_xxx.c
index 9af4823..42d2d48 100644
--- a/mingw-w64-crt/testcases/tstmain_sys_xxx.c
+++ b/mingw-w64-crt/testcases/tstmain_sys_xxx.c
@@ -4,8 +4,7 @@
  * No warranty is given; refer to the file DISCLAIMER.PD within this package.
  */
 #include <stdio.h>
-
-_CRTIMP char *_sys_errlist[];
+#include <stdlib.h>
 
 int main(int argc, char **argv)
 {