common/ceph_context.h: Jenkins builder fix: breakpad reserve space

common/ceph_context.h: Jenkins builder fix: breakpad reserve space

For cases when HAVE_BREAKPAD is off, supply exactly the same space in
CephContext struct.

While it should happen, jenkins seems to link binaries with different variants.

The noticeable artefacts of this misbehaviour are:
	208 - unittest_bluefs (Bus error)
	209 - unittest_bluefs_ex (Failed)
	211 - unittest_bdev (Bus error)

Above mentioned unittests are failing because
ceph_context.h :

  ceph::PluginRegistry *get_plugin_registry() {
    return _plugin_registry;
  }
^ _plugin_registry returned is at !!!offset off by 8 bytes!!! to the location of _plugin_registry as constructed at
ceph_context.cc :

743:   _plugin_registry = new PluginRegistry(this);

This causes fatal error in
src/extblkdev/ExtBlkDevPlugin.cc :

227      auto registry = cct->get_plugin_registry();
228      std::lock_guard l(registry->lock);

Sometimes lock_guard hangs, sometimes lock_guard segfaults.

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>