Add <sys/select.h> where appropriate

Under Posix systems, this header is documented to be required
when using select().

src/ser_posix.c

@@ -36,6 +36,7 @@
 #include <sys/ioctl.h>
 #include <sys/types.h>
 #include <sys/time.h>
+#include <sys/select.h>
 #include <sys/socket.h>
 #include <netdb.h>
 

src/term.c

@@ -44,6 +44,8 @@
 
 #ifdef WIN32
 #include <windows.h>
+#else
+#include <sys/select.h>
 #endif
 #endif