Add include for pwd.h

This was previously included via the PostgreSQL source, but that
seems to have gone away in recent HEAD builds.

repmgr-client.c

@@ -51,6 +51,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <pwd.h>
 #include <unistd.h>
 #include <sys/stat.h>
 #include <signal.h>