change firstByteMark to unsigned to prevent constant overflow warning

git-svn-id: http://svn.code.sf.net/p/cjson/code@33 e3330c51-1366-4df0-8b21-3ccf24e3d50e

cJSON.c

@@ -135,7 +135,7 @@ static char *print_number(cJSON *item)
 }
 
 // Parse the input text into an unescaped cstring, and populate item.
-static const char firstByteMark[7] = { 0x00, 0x00, 0xC0, 0xE0, 0xF0, 0xF8, 0xFC };
+static const unsigned char firstByteMark[7] = { 0x00, 0x00, 0xC0, 0xE0, 0xF0, 0xF8, 0xFC };
 static const char *parse_string(cJSON *item,const char *str)
 {
 	const char *ptr=str+1;char *ptr2;char *out;int len=0;unsigned uc;