doc: add the corresponding @endcond command for @cond

* they are used to applease asphyxiate, as it
  is not able to handle "enum" sections

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/include/rados/librados.h

@@ -81,6 +81,7 @@ enum {
   // indicate read/write data will not accessed again (by *this* client)
   LIBRADOS_OP_FLAG_FADVISE_NOCACHE    = 0x40,
 };
+/** @endcond */
 
 #if __GNUC__ >= 4
   #define CEPH_RADOS_API  __attribute__ ((visibility ("default")))