Add missing macOS framework in echo_server.c

[mastermakrela]

Currently (at least on macOS Sequoia 15.2) the example/echo_server.c doesn't build with the following error:

cc -framework CoreFoundation -framework Security echo_server.c ../libtailscale.a
Undefined symbols for architecture arm64:
  "_IOObjectRelease", referenced from:
      _getSerialNumber in libtailscale.a[17](000014.o)
  "_IORegistryEntryCreateCFProperty", referenced from:
      _getSerialNumber in libtailscale.a[17](000014.o)
  "_IOServiceGetMatchingService", referenced from:
      _getSerialNumber in libtailscale.a[17](000014.o)
  "_IOServiceMatching", referenced from:
      _getSerialNumber in libtailscale.a[17](000014.o)
  "_kIOMainPortDefault", referenced from:
      _getSerialNumber in libtailscale.a[17](000014.o)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I could trace this problem to missing -framework IOKit in the hint about building on macOS.
So to save time for others who might also be trying this example, it would be nice to add it.

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I couldn't find any contributing guide in this repository, so sorry in advance if I made any mistakes — just let me know, and I'll fix them. :)