BUILD: compression: remove a warning when no compression lib is used

BUILD: compression: remove a warning when no compression lib is used

This warning appears when building without any compression library :

src/compression.c:143:19: warning: unused function 'init_comp_ctx' [-Wunused-function]
static inline int init_comp_ctx(struct comp_ctx **comp_ctx)
                  ^
src/compression.c:176:19: warning: unused function 'deinit_comp_ctx' [-Wunused-function]
static inline int deinit_comp_ctx(struct comp_ctx **comp_ctx)

No backport is needed.