[ruby/date] Update zonetab.h at 2024-06-26

https://github.com/ruby/date/commit/ef5a0dac5b

ext/date/zonetab.h

@@ -51,7 +51,7 @@ struct zone;
 
 #ifndef GPERF_DOWNCASE
 #define GPERF_DOWNCASE 1
-static const unsigned char gperf_downcase[256] =
+static unsigned char gperf_downcase[256] =
   {
       0,   1,   2,   3,   4,   5,   6,   7,   8,   9,  10,  11,  12,  13,  14,
      15,  16,  17,  18,  19,  20,  21,  22,  23,  24,  25,  26,  27,  28,  29,