Make use of methods defined in utils unix layer

Some unix-like concepts are supported by VxWorks VSB layer - UTILS_UNIX.
One of such methods is getgrgid(). Include it in
`qfilesystemengine_unix.cpp`, so that we don't need to exclude VxWorks from code that uses it anymore.

Task-number: QTBUG-115777
Change-Id: I72b301647bfdb208cb6859bb0f9994e3537fc345
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 21196d26d84f0f9984308660714921aa17790bc4)

src/corelib/io/qfilesystemengine_unix.cpp

@@ -19,6 +19,7 @@
 # include <QtCore/private/qtemporaryfile_p.h>
 #endif // QT_BOOTSTRAPPED
 
+#include <grp.h>
 #include <pwd.h>
 #include <stdlib.h> // for realpath()
 #include <unistd.h>