fix conflicting types for size and _size

gcc -Wall -Wextra -g -fsanitize=address -o spell_check dictionary.c spell_check.c

dictionary.c:68:5: error: conflicting types for 'size'
   68 | int size(void) {
      |     ^~~~
In file included from dictionary.c:6:
dictionary.h:9:14: note: previous declaration of 'size' was here
    9 | unsigned int size(void);
      |              ^~~~

michaelg29 / yt-challenges

fix conflicting types for size and _size #6