libcephfs.h: fix unnecessary forward declarations

libcephfs.h: fix unnecessary forward declarations

This is an issue in case non-cpp code. Move the declaration of
inodeno_t and vinodeno_t structs into __cplusplus block where
they are needed.

Fix for cppcheck issue:

[src/include/cephfs/libcephfs.h:89] -> [src/include/cephfs/libcephfs.h:74]:
 The struct 'inodeno_t' forward declaration is unnecessary. Type struct is
 already declared earlier.
[src/include/cephfs/libcephfs.h:93] -> [src/include/cephfs/libcephfs.h:83]:
 The struct 'vinodeno_t' forward declaration is unnecessary. Type struct is
 already declared earlier.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>