apps: Move libevent dependency to src/apps/meson.build

libevent is a shared dependency between cam and lc-compliance, move it to
src/apps/. The shared dependency will be used to condition compilation
of source files in an upcoming application static library.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/apps/cam/meson.build

@@ -1,8 +1,6 @@
 # SPDX-License-Identifier: CC0-1.0
 
-libevent = dependency('libevent_pthreads', required : get_option('cam'))
-
-if not libevent.found()
+if opt_cam.disabled() or not libevent.found()
     cam_enabled = false
     subdir_done()
 endif