[ruby/nkf] Constified invariant tables

https://github.com/ruby/nkf/commit/b386ddc11c

ext/nkf/nkf-utf8/nkf.c

@@ -144,7 +144,7 @@ static void w_oconv(nkf_char c2, nkf_char c1);
 static void w_oconv16(nkf_char c2, nkf_char c1);
 static void w_oconv32(nkf_char c2, nkf_char c1);
 
-typedef struct {
+typedef const struct {
     const char *name;
     nkf_char (*iconv)(nkf_char c2, nkf_char c1, nkf_char c0);
     void (*oconv)(nkf_char c2, nkf_char c1);
@@ -158,10 +158,10 @@ nkf_native_encoding NkfEncodingUTF_8 =		{ "UTF-8", w_iconv, w_oconv };
 nkf_native_encoding NkfEncodingUTF_16 =		{ "UTF-16", w_iconv16, w_oconv16 };
 nkf_native_encoding NkfEncodingUTF_32 =		{ "UTF-32", w_iconv32, w_oconv32 };
 
-typedef struct {
-    const int id;
+typedef const struct {
+    int id;
     const char *name;
-    const nkf_native_encoding *base_encoding;
+    nkf_native_encoding *base_encoding;
 } nkf_encoding;
 
 nkf_encoding nkf_encoding_table[] = {
@@ -204,9 +204,9 @@ nkf_encoding nkf_encoding_table[] = {
     {-1,		NULL,			NULL}
 };
 
-struct {
+static const struct {
     const char *name;
-    const int id;
+    int id;
 } encoding_name_to_id_table[] = {
     {"US-ASCII",		ASCII},
     {"ASCII",			ASCII},
@@ -4286,7 +4286,7 @@ static const unsigned char *mime_pattern[] = {
 
 
 /* $B3:Ev$9$k%3!<%I$NM%@hEY$r>e$2$k$?$a$NL\0u(B */
-nkf_char (*mime_priority_func[])(nkf_char c2, nkf_char c1, nkf_char c0) = {
+static nkf_char (*const mime_priority_func[])(nkf_char c2, nkf_char c1, nkf_char c0) = {
     e_iconv, s_iconv, 0, 0, 0, 0, 0,
 #if defined(UTF8_INPUT_ENABLE)
     w_iconv, w_iconv,