ceph: Fix compilation with GCC compiler

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
libcephfs: Fix compilation for C compiler

Use struct instead of class, even if it warns.

Warnings are lower priority than compilation break.

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
test: add gcc compile test for installed headers

libcephfs/librados headers should work with gcc.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #797 from ceph/wip-6680

Wip 6680

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>