fix compiler warning

src/shared/utf8.c:268:13: warning: unused variable 'd'
[-Wunused-variable]
         int d;

src/shared/utf8.c

@@ -265,7 +265,6 @@ char *ascii_is_valid(const char *str) {
 
 int utf8_encode_unichar(uint16_t c, char *p) {
         uint8_t *t = (uint8_t*) p;
-        int d;
 
         if (c < 0x80) {
                 t[0] = (uint8_t) c;