src/parser.c: declare ohcount_hash_parser_from_language() to fix build error with...

src/parser.c: declare ohcount_hash_parser_from_language() to fix build error with modern gcc versions

The function ohcount_hash_parser_from_language() was not declared,
resulting in the following compiler error with gcc 4.4.3:

  src/parser.c: In function ‘ohcount_parse’:
  src/parser.c:17: warning: implicit declaration of function ‘ohcount_hash_parser_from_language’
  src/parser.c:17: warning: initialization makes pointer from integer without a cast
  src/parser.c:22: error: dereferencing pointer to incomplete type