#6 include unistd for close declaration

[Thomas Perale]

When using gcc-14 the compilation fails with the following error
message:

close.c: In function 'EIBClose':
close.c:38:3: error: implicit declaration of function 'close'; did you mean 'pclose'? [-Wimplicit-function-declaration]
   38 |   close (con->fd);
      |   ^~~~~
      |   pclose
openlocal.c: In function 'EIBSocketLocal':
openlocal.c:57:7: error: implicit declaration of function 'close'; did you mean 'pclose'? [-Wimplicit-function-declaration]
   57 |       close (con->fd);
      |       ^~~~~
      |       pclose
make[6]: *** [Makefile:611: close.lo] Error 1

This patch include <unistd.h> to include the declaration of close.</unistd.h>