acinclude.m4: fix -Wimplicit-function-declaration

-Wimplicit-function-declaration will become an error by default
in Clang 16.

Fixes errors like:
```
error: call to undeclared library function 'exit' with type 'void (int) __attribute__((noreturn))'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
```

Signed-off-by: Sam James <sam@gentoo.org>

acinclude.m4

@@ -471,6 +471,7 @@ AC_DEFUN([SANE_CHECK_IPV6],
   if test "$ipv6" != "no" ; then
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 	#define INET6
+	#include <stdlib.h>
 	#include <sys/types.h>
 	#include <sys/socket.h> ]], [[
 	 /* AF_INET6 available check */
@@ -492,6 +493,7 @@ AC_DEFUN([SANE_CHECK_IPV6],
     AC_MSG_CHECKING([whether struct sockaddr_storage has an ss_family member])
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 	#define INET6
+	#include <stdlib.h>
 	#include <sys/types.h>
 	#include <sys/socket.h> ]], [[
 	/* test if the ss_family member exists in struct sockaddr_storage */
@@ -504,6 +506,7 @@ AC_DEFUN([SANE_CHECK_IPV6],
     ], [
 		AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 		#define INET6
+		#include <stdlib.h>
 		#include <sys/types.h>
 		#include <sys/socket.h> ]], [[
 		/* test if the __ss_family member exists in struct sockaddr_storage */