Android: Fix compile on arm64-v8a

Android arm64-v8a redefines _POSIX_C_SOURCE to 199506 if _XOPEN_SOURCE is defined.

Change-Id: I04ff616eaac6e08094fc1f58f49bc5a2c31733f0
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

src/corelib/io/forkfd_qt.cpp

@@ -35,7 +35,7 @@
 #ifndef _POSIX_C_SOURCE
 #  define _POSIX_C_SOURCE 200809L
 #endif
-#if !defined(_XOPEN_SOURCE) && !defined(__QNXNTO__)
+#if !defined(_XOPEN_SOURCE) && !defined(__QNXNTO__) && !defined(ANDROID)
 #  define _XOPEN_SOURCE 500
 #endif