core/logger.c: mark compilation-unit private variables static

core/logger.c: mark compilation-unit private variables static

Fixes compiler diagnostics:

logger.c:29:6: warning: no previous extern declaration for non-static
    variable '_mowgli_log_buf' [-Wmissing-variable-declarations]

logger.c:30:17: warning: no previous extern declaration for non-static
    variable '_mowgli_log_cb' [-Wmissing-variable-declarations]

logger.c:33:1: warning: no previous prototype for function
    'mowgli_log_cb_default' [-Wmissing-prototypes]