fcaps.eclass: don't assume D and ROOT end with a slash

BUG=b/179427028
TEST=presubmit
RELEASE_NOTE=None

Closes: https://bugs.gentoo.org/655244
Change-Id: I78c00d84456226e5b4ff534fcd2ce52a36c0f1fa
Reviewed-on: https://cos-review.googlesource.com/c/third_party/overlays/portage-stable/+/13216
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
diff --git a/eclass/fcaps.eclass b/eclass/fcaps.eclass
index 8ce5b96..822114a 100644
--- a/eclass/fcaps.eclass
+++ b/eclass/fcaps.eclass
@@ -111,11 +111,12 @@
 		root=${EROOT:-${ROOT}}
 		;;
 	esac
+	root=${root%/}
 
 	# Process every file!
 	local file
 	for file ; do
-		[[ ${file} != /* ]] && file="${root}${file}"
+		[[ ${file} != /* ]] && file="${root}/${file}"
 
 		if use filecaps ; then
 			# Try to set capabilities.  Ignore errors when the