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]

src/libmowgli/core/logger.c

@@ -26,10 +26,10 @@
 
 #define MOWGLI_LOG_BUF_SIZE 4096
 
-char _mowgli_log_buf[MOWGLI_LOG_BUF_SIZE];
-mowgli_log_cb_t _mowgli_log_cb;
+static char _mowgli_log_buf[MOWGLI_LOG_BUF_SIZE];
+static mowgli_log_cb_t _mowgli_log_cb;
 
-void
+static void
 mowgli_log_cb_default(const char *buf)
 {
 	fprintf(stderr, "%s\n", buf);