allow shadow functions to be called from modules

Fixes an issue where attempting to call the shadow functions from a
module results in modpost failures:

    ERROR: "kpatch_shadow_get" [net/mac80211/mac80211.ko] undefined!
    ERROR: "kpatch_shadow_alloc" [net/mac80211/mac80211.ko] undefined!

LDFLAGS_MODULE is apparently not really needed.
Merge pull request #441 from jpoimboe/module-shadow

allow shadow functions to be called from modules