Added static modifier to private logger variables

* This prevents the user from referencing the variables using the extern 
keyword and them tampering with them;

src/message_logger.c

@@ -4,9 +4,9 @@
 #include "message_logger.h"
 
 // Private variables:
-char time_fmt[TIME_FMT_SIZE] = "%H:%M:%S %d-%m-%Y";
-FILE *log_file = NULL;
-pthread_mutex_t *logger_recursive_mutex = NULL;
+static char time_fmt[TIME_FMT_SIZE] = "%H:%M:%S %d-%m-%Y";
+static FILE *log_file = NULL;
+static pthread_mutex_t *logger_recursive_mutex = NULL;
 
 // Private function headers:
 static void apply_all_default_attributes();