add sys/select.h for musl libc

Author: aguadoenzo

httplib.h

@@ -61,6 +61,7 @@ typedef int socket_t;
 #include <string>
 #include <thread>
 #include <sys/stat.h>
+#include <sys/select.h>
 #include <fcntl.h>
 #include <assert.h>