Include unistd.h in the unix implementation source of QLockFile

This is necessary to avoid hidden dependencies for int gethostname(char *name,
size_t namelen);

This was revelead while backporting the class to Qt 4 for QtSerialPort.

Change-Id: I1c477d295eeae90b3136fc6aae5d45982d5b1d73
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qlockfile_unix.cpp

@@ -54,6 +54,7 @@
 #include <sys/file.h>  // flock
 #include <sys/types.h> // kill
 #include <signal.h>    // kill
+#include <unistd.h>    // gethostname
 
 QT_BEGIN_NAMESPACE