testsuite: stdlib/isomac.c: add missing include

When running the testsuite, building stdlib/isomac.c outputs the
following warning:

  gcc -O   -D_GNU_SOURCE -DIS_IN_build -include /home/aurel32/glibc-build/config.h isomac.c -o /home/aurel32/glibc-build/stdlib/isomac
  isomac.c: In function ‘get_null_defines’:
  isomac.c:260:3: warning: implicit declaration of function ‘close’; did you mean ‘pclose’? [-Wimplicit-function-declaration]
     close (fd);
     ^~~~~
     pclose

Fix that by adding the <unistd.h> include.

Changelog:
	* stdlib/isomac.c: Include <unistd.h>.
(cherry picked from commit 11f382ee78)

ChangeLog

@@ -1,3 +1,7 @@
+2019-02-03  Aurelien Jarno  <aurelien@aurel32.net>
+
+	* stdlib/isomac.c: Include <unistd.h>.
+
 2018-11-07  Florian Weimer  <fweimer@redhat.com>
 
 	Implement TEST_COMPARE_STRING.

stdlib/isomac.c

@@ -74,6 +74,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 #define HEADER_MAX          256