gptcurses: Add missing header

Needed with uClibc++ as it doesn't implicitly include it.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
diff --git a/gptcurses.cc b/gptcurses.cc
index 3e9b240..4ebfde1 100644
--- a/gptcurses.cc
+++ b/gptcurses.cc
@@ -19,6 +19,7 @@
  *
  */
 
+#include <clocale>
 #include <iostream>
 #include <string>
 #include <sstream>