[ruby/date] Constify gperf-generated table

https://github.com/ruby/date/commit/6d7ab08ffc

ext/date/zonetab.h

@@ -49,7 +49,7 @@ struct zone;
 
 #ifndef GPERF_DOWNCASE
 #define GPERF_DOWNCASE 1
-static unsigned char gperf_downcase[256] =
+static const 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,

tool/gperf.sed

@@ -20,3 +20,4 @@ s/(int)([a-z_]*)&((struct \([a-zA-Z_0-9][a-zA-Z_0-9]*\)_t *\*)0)->\1_str\([1-9][
   s/ hval = / hval = (unsigned int)/
   s/ return / return (unsigned int)/
 }
+s/^\(static\) \(unsigned char gperf_downcase\)/\1 const \2/