build: avoid compiler warning on cygwin

build: avoid compiler warning on cygwin

On cygwin:

  CC       libvirt_driver_security_la-security_dac.lo
security/security_dac.c: In function 'virSecurityDACSetProcessLabel':
security/security_dac.c:618: warning: format '%d' expects type 'int', but argument 7 has type 'uid_t' [-Wformat]

We've done this before (see src/util/util.c).

* src/security/security_dac.c (virSecurityDACSetProcessLabel): On
cygwin, uid_t is a 32-bit long.