Include <sys/type.h> before defining "ushort" macro

sys/types.h uses a typedef to define ushort. If ushort is defined as a macro,
this will cause a compile failure.

In file included from serdisp_control.c:43:
../include/serdisplib/serdisp_gpevents.h:75:19: error: expected identifier or '(' before 'unsigned'
   75 |   #define ushort (unsigned short)

--- a/include/serdisplib/serdisp_gpevents.h
+++ b/include/serdisplib/serdisp_gpevents.h
@@ -50,6 +50,7 @@
 #include "serdisplib/serdisp_connect.h"
 #include "serdisplib/serdisp_control.h"
 #include <sys/time.h>
+#include <sys/types.h>
 #include <stdint.h>
 
 #include "../../config.h"