elua - fix build for luajit2.1.0-beta3+

it seems luajit (and lua) broke api again... removed a #define from
their headers from 5.1->5.2 of lua (and seemingly luajit2.1 too). :(

this should fix T2728

@fix

src/lib/elua/elua_private.h

@@ -28,6 +28,11 @@
 #include <lualib.h>
 #include <lauxlib.h>
 
+// FIX for luajit 2.1.0-beta3: see https://phab.enlightenment.org/T2728
+#ifndef luaL_reg
+# define luaL_reg luaL_Reg
+#endif
+
 #include "Elua.h"
 
 struct _Elua_State