feat(solutions): add fibonacci/c/function

challenges/fibonacci/solutions/c/function/README.md

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+# fibonacci/c/function
+
+Created by [@Divlo](https://github.com/Divlo) on 29 September 2021.

challenges/fibonacci/solutions/c/function/solution.c

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+#include <stdio.h>
+#include <stdlib.h>
+
+int fibonacci(int number) {
+  return number < 2 ? number : fibonacci(number - 1) + fibonacci(number - 2);
+}
+
+int main() {
+  int number;
+  scanf("%d", &number);
+  printf("%d\n", fibonacci(number));
+  return EXIT_SUCCESS;
+}