build: avoid compilation warnings

build: avoid compilation warnings

Detected on cygwin:
util/util.c: In function 'virSetUIDGID':
util/util.c:2824: warning: format '%d' expects type 'int', but argument 7 has type 'gid_t' [-Wformat]
(and three other lines)

* src/util/util.c (virSetUIDGID): Cast, as is done elsewhere in
this file, to avoid printf type mismatch warnings.