build: fix mingw build

build: fix mingw build

./autobuild.sh died on several messages resembling:

../../src/rpc/virnetsocket.c: In function 'virNetSocketNewListenTCP':
../../src/rpc/virnetsocket.c:231:9: error: implicit declaration of function 'bind_used_without_requesting_gnulib_module_bind' [-Wimplicit-function-declaration]
../../src/rpc/virnetsocket.c:231:9: error: nested extern declaration of 'bind_used_without_requesting_gnulib_module_bind' [-Wnested-externs]

Basically, gnulib socket fds are not safe to pass to mingw socket
functions unless we pull in those gnulib modules.

* bootstrap.conf (gnulib_modules): Add modules to handle socket
functions on mingw.