Fix SYSCONFDIR to include "prefix"

SYSCONFDIR is used to determine the path of the default configuration
file. 'sysconfdir' is set to 'prefix/sysconfdir' later (on line 139),
so configuration files are installed under 'prefix', but SYSCONFDIR did
not reflect it.

meson.build

@@ -104,7 +104,7 @@ if scdoc.found()
 	endforeach
 endif
 
-add_project_arguments('-DSYSCONFDIR="/@0@"'.format(sysconfdir), language : 'c')
+add_project_arguments('-DSYSCONFDIR="/@0@/@1@"'.format(prefix, sysconfdir), language : 'c')
 
 version = get_option('sway_version')
 if version != ''