build: depend on epoll-shim on non-Linux after 528eb7e

ld: error: undefined symbol: epoll_ctl
>>> referenced by steamcompmgr.cpp
>>>               src/gamescope.p/steamcompmgr.cpp.o:(update_wayland_res(CommitDoneList_t*, steamcompmgr_win_t*, ResListEntry_t&))
>>> referenced by steamcompmgr.cpp
>>>               src/gamescope.p/steamcompmgr.cpp.o:(commit_t::OnPollIn())
>>> referenced by steamcompmgr.cpp
>>>               src/gamescope.p/steamcompmgr.cpp.o:(gamescope::CWaiter<1024ul>::CWaiter())

ld: error: undefined symbol: epoll_create1
>>> referenced by steamcompmgr.cpp
>>>               src/gamescope.p/steamcompmgr.cpp.o:(gamescope::CWaiter<1024ul>::CWaiter())

ld: error: undefined symbol: epoll_wait
>>> referenced by steamcompmgr.cpp
>>>               src/gamescope.p/steamcompmgr.cpp.o:(gamescope::CAsyncWaiter<1024ul>::WaiterThreadFunc(char const*))

src/meson.build

@@ -15,6 +15,7 @@ wayland_protos = dependency('wayland-protocols', version: '>=1.17')
 xkbcommon = dependency('xkbcommon')
 thread_dep = dependency('threads')
 cap_dep = dependency('libcap', required: false)
+epoll_dep = dependency('epoll-shim', required: false)
 glm_dep = dependency('glm')
 sdl_dep = dependency('SDL2')
 stb_dep = dependency('stb')
@@ -132,7 +133,7 @@ endif
       dep_wayland, dep_x11, dep_xdamage, dep_xcomposite, dep_xrender, dep_xext, dep_xfixes,
       dep_xxf86vm, dep_xres, glm_dep, drm_dep, wayland_server,
       xkbcommon, thread_dep, sdl_dep, wlroots_dep,
-      vulkan_dep, liftoff_dep, dep_xtst, dep_xmu, cap_dep, pipewire_dep, librt_dep,
+      vulkan_dep, liftoff_dep, dep_xtst, dep_xmu, cap_dep, epoll_dep, pipewire_dep, librt_dep,
       stb_dep, displayinfo_dep, openvr_dep,
     ],
     install: true,