Use actual constants for masks

Author: byroot

ext/json/ext/vendor/jeaiii-ltoa.h

@@ -81,9 +81,9 @@ static struct pair digits_fd[100] = {
 
 #undef NUL
 
-static u64_t mask24 = (u64(1) << 24) - 1;
-static u64_t mask32 = (u64(1) << 32) - 1;
-static u64_t mask57 = (u64(1) << 57) - 1;
+static const u64_t mask24 = (u64(1) << 24) - 1;
+static const u64_t mask32 = (u64(1) << 32) - 1;
+static const u64_t mask57 = (u64(1) << 57) - 1;
 
 static
 char* to_text_from_ulong(char* b, u64_t n)