virfile: Fix build with glibc 2.36

virfile: Fix build with glibc 2.36

With glibc 2.36, sys/mount.h and linux/mount.h conflict:
https://sourceware.org/glibc/wiki/Release/2.36#Usage_of_.3Clinux.2Fmount.h.3E_and_.3Csys.2Fmount.h.3E

virfile.c imports sys/mount.h and linux/fs.h, which pulls in
linux/mount.h.

Manually define the constants we need from linux/fs.h, like was
done in llvm:

https://reviews.llvm.org/rGb379129c4beb3f26223288627a1291739f33af02

Reviewed-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>