AUTHORS.h: Global variables should be const.

Non-const global variables should not be used

AUTHORS.h

@@ -10,7 +10,7 @@
 #pragma once
 
 //VCMI PROJECT CODE CONTRIBUTORS:
-std::vector<std::vector<std::string>> contributors = {
+const std::vector<std::vector<std::string>> contributors = {
 //   Task          Name                    Aka                      E-Mail
    { "Idea",       "Michał Urbańczyk",     "Tow",                   "[email protected]"             },
    { "Idea",       "Mateusz B.",           "Tow dragon",            "[email protected]"            },