Added Learning GNU C book (#4645)

* Added Learning GNU C book -  check_urls=free-programming-books.md

* Added Learning GNU C book - check_urls=free-programming-books.md

free-programming-books.md

@@ -1041,6 +1041,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Functional C (1997)](https://research.utwente.nl/files/5128727/book.pdf) - Pieter H. Hartel, Henk Muller (PDF)
 * [Hashing](https://www.smashwords.com/books/view/737188) - Prakash Hegade
 * [Learn to Code With C - The MagPi Essentials](https://www.raspberrypi.org/magpi-issues/Essentials_C_v1.pdf) (PDF)
+* [Learning GNU C](https://download-mirror.savannah.gnu.org/releases/c-prog-book/learning_gnu_c.pdf) - Ciaran O’Riordan (PDF)
 * [Modeling with Data](https://ben.klemens.org/pdfs/gsl_stats.pdf) - Ben Klemens (PDF)
 * [Modern C](https://modernc.gforge.inria.fr) - Jens Gustedt (PDF)
 * [Object-Oriented Programming With ANSI-C](http://www.planetpdf.com/codecuts/pdfs/ooc.pdf) (PDF)