feat(solutions): add single-number/c/function

challenges/single-number/solutions/c/function/README.md

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+# single-number/c/function
+
+Created by [@theoludwig](https://github.com/theoludwig) on 21 August 2023.

challenges/single-number/solutions/c/function/solution.c

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+#include <stdio.h>
+#include <stdlib.h>
+
+int single_number(int* numbers, size_t numbers_size) {
+  int result = numbers[0];
+  for (size_t index = 1; index < numbers_size; index++) {
+    result = result ^ numbers[index];
+  }
+  return result;
+}
+
+int main() {
+  size_t numbers_size = 0;
+  int number;
+  int* numbers = malloc(sizeof(int));
+  do {
+    scanf("%d", &number);
+    numbers = realloc(numbers, sizeof(int) * (numbers_size + 1));
+    numbers[numbers_size] = number;
+    numbers_size++;
+  } while (fgetc(stdin) != EOF);
+  printf("%d\n", single_number(numbers, numbers_size));
+  free(numbers);
+  return EXIT_SUCCESS;
+}