Make modbus.h more self contained

If <sys/time.h> is not included before "modbus.h" as struct timeval
is not declared. This behavior was observed while building libmodbus
for QNX.

Signed-off-by: Matthias Weisser <weisserm@arcor.de>

src/modbus.c

@@ -38,7 +38,6 @@
 #include <stdint.h>
 #endif
 #include <termios.h>
-#include <sys/time.h>
 #include <unistd.h>
 #include <errno.h>
 #include <assert.h>

src/modbus.h

@@ -37,6 +37,7 @@
 #include <netinet/ip.h>
 #include <netinet/tcp.h>
 #include <arpa/inet.h>
+#include <sys/time.h>
 
 #include "modbus-version.h"