common/lockdep: explicitly export g_lockdep visibility

common/lockdep: explicitly export g_lockdep visibility

Aggressive Link-Time Optimization (LTO) and stricter linker defaults
in newer toolchains (like GCC 13 on Ubuntu 24.04) can strip or
localize external global variables if their visibility is not
explicitly defined. Tagging g_lockdep with default visibility ensures
it remains accessible in the dynamic symbol table for external
binaries like monmaptool.

Assisted-by: Gemini
Fixes: https://tracker.ceph.com/issues/78321
Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>