verify: document conflict with -Wnested-externs

verify: document conflict with -Wnested-externs

Our implementation of verify() relies on nested extern declarations,
so it is incompatible with gcc < 4.6 coupled with -Wnested-externs
when used inside a function body.  But inside a function body, you
can always arrange to use verify_expr() instead, which is warning-free.

* lib/verify.h: Give hint about usage when gcc warnings are enabled.

Signed-off-by: Eric Blake <eblake@redhat.com>