Add missing keygen.c

Signed-off-by: Adrien Gallouët <adrien@gallouet.fr>

src/keygen.c

@@ -0,0 +1,22 @@
+#include "common.h"
+
+#include <sodium.h>
+#include <stdio.h>
+
+int
+gt_keygen(int argc, char **argv)
+{
+    if (sodium_init() == -1) {
+        gt_log("sodium init failed\n");
+        return 1;
+    }
+
+    unsigned char key[32];
+    randombytes_buf(key, sizeof(key));
+
+    char buf[2 * sizeof(key) + 1];
+    gt_tohex(buf, sizeof(buf), key, sizeof(key));
+    printf("%s\n", buf);
+
+    return 0;
+}