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>.

1 files changed, 1 insertions, 0 deletions

diff --git a/stdlib/isomac.c b/stdlib/isomac.c
index 37f564c82a..7d743cb8e5 100644
--- a/stdlib/isomac.c
+++ b/stdlib/isomac.c
@@ -74,6 +74,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 #define HEADER_MAX          256