libstore: Include missing header to fix compile with libc++ 20

https://en.cppreference.com/w/cpp/thread.html

src/libstore/gc.cc:121:39: error: no member named 'sleep_for' in namespace 'std::this_thread'
  121 |                     std::this_thread::sleep_for(std::chrono::milliseconds(100));
      |                     ~~~~~~~~~~~~~~~~~~^

(cherry picked from commit 140b08ae3e)

src/libstore/gc.cc

@@ -22,6 +22,7 @@
 #include <random>
 
 #include <climits>
+#include <thread>
 #include <errno.h>
 #include <fcntl.h>
 #include <sys/stat.h>