feat(solutions): add first-non-repeating-character/c/function

challenges/first-non-repeating-character/solutions/c/function/solution.c

@@ -0,0 +1,19 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "character.h"
+#include "input.h"
+#include "string.h"
+
+int main() {
+  char* string = input();
+  char character = string_first_non_repeating_character(string);
+  if (character == 0) {
+    printf("\n");
+  } else {
+    printf("%c\n", character);
+  }
+  free(string);
+  return EXIT_SUCCESS;
+}