Fix feature test macro

The expected feature test macro is _POSIX_C_SOURCE, not _POSIX_SOURCE.

Fixes #1726 <https://github.com/Genymobile/scrcpy/issues/1726>

app/src/server.c

@@ -201,7 +201,7 @@ enable_tunnel_forward_any_port(struct server *server,
 
         if (port < port_range.last) {
             LOGW("Could not forward port %" PRIu16", retrying on %" PRIu16,
-                 port, port + 1);
+                 port, (uint16_t) (port + 1));
             port++;
             continue;
         }

app/src/sys/unix/command.c

@@ -1,6 +1,6 @@
-// for portability
+// for portability (kill, readlink, strdup, strtok_r)
-#define _POSIX_SOURCE // for kill()
+#define _POSIX_C_SOURCE 200809L
-#define _BSD_SOURCE // for readlink()
+#define _BSD_SOURCE
 
 // modern glibc will complain without this
 #define _DEFAULT_SOURCE