[libc] Consume the %s format character in vsprintf

Previously we were outputing a single 's' after the string.

lib/libc/src/stdio.cpp

@@ -111,6 +111,7 @@ int vsprintf(char *str, const char *format, va_list arg) {
           *(str++) = *(instr++);
           width++;
         }
+        format++;
         break;
       }
       default: