Move code from jid_random_resource() into own function

Move the code that creates a random string into it's own function
+get_random_string().

src/common.h

@@ -103,4 +103,6 @@ int is_regular_file(const char *path);
 int is_dir(const char *path);
 void get_file_paths_recursive(const char *directory, GSList **contents);
 
+char* get_random_string(int length);
+
 #endif