Make sure that the generator is only active on merged systems

There are rpms which have files in /usr/bin which would trigger the
condition in the generator, even when building on an unmerged system,
and the package would have Requires:filesystem(unmerged-sbin-symlinks).
The corresponding Provides is actually only generated when filesystem
itself is built with the merge, so effectively the package with the
Requires would become uninstallable.

filesystem.req

@@ -7,6 +7,10 @@
 # this out and we need a hardcoded list.
 
 sbin_filenames=${1:?}
+
+# Do nothing if the build system is not merged.
+test -L /usr/sbin || exit 0
+
 declare -A "filenames=($(sed 's/.*/["\0"]=1/' <"${sbin_filenames}"))"
 
 while read -r path; do