Fix clang warnings about implicit declaration

src/modbus-rtu.c

@@ -20,6 +20,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <string.h>
+#include <unistd.h>
 
 #include "modbus.h"
 #include "modbus-private.h"
@@ -250,6 +251,8 @@ ssize_t _modbus_rtu_recv(modbus_t *ctx, uint8_t *rsp, int rsp_length)
 #endif
 }
 
+int _modbus_rtu_flush(modbus_t *);
+
 /* The check_crc16 function shall return the message length if the CRC is
    valid. Otherwise it shall return -1 and set errno to EMBADCRC. */
 int _modbus_rtu_check_integrity(modbus_t *ctx, uint8_t *msg,

src/modbus-tcp.c

@@ -22,6 +22,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <unistd.h>
 
 #include <sys/types.h>
 #ifdef NATIVE_WIN32