build: simplify use of verify

build: simplify use of verify

Back in 2008 when this line of util.h was written, gnulib's verify
module didn't allow the use of multiple verify() in one file
in combination with our choice of gcc -W options.  But that has
since been fixed in gnulib, and newer gnulib even maps verify()
to the C1x feature of _Static_assert, which gives even nicer
diagnostics with a new enough compiler, so we might as well go
with the simpler verify().

* src/util/util.h (VIR_ENUM_IMPL): Use simpler verify, now that
gnulib module is smarter.