Make libcap an optional dependency

FreeBSD does not have libcap, so without "required: false" Sway fails
to build.

meson.build

@@ -38,7 +38,7 @@ pango          = dependency('pango')
 pangocairo     = dependency('pangocairo')
 gdk_pixbuf     = dependency('gdk-pixbuf-2.0', required: false)
 pixman         = dependency('pixman-1')
-libcap         = dependency('libcap')
+libcap         = dependency('libcap', required: false)
 libinput       = dependency('libinput', version: '>=1.6.0')
 libpam         = cc.find_library('pam')
 systemd        = dependency('libsystemd', required: false)