Dpkg::Source::Package::V2: Make the BinaryFiles package self-contained

Explicitly import File::Spec, to avoid relying on the other imports in
the same file.

debian/changelog

@@ -19,6 +19,8 @@ dpkg (1.18.2) UNRELEASED; urgency=low
     - Allow the Maintainer field in CTRL_FILE_STATUS.
     - Import make_path from File::Path in Dpkg::Source::Package::V2.
       Regression introduced in dpkg 1.18.0. Closes: #789957
+    - Make the BinaryFiles subpackage self-contained by explicitly importing
+      File::Spec in Dpkg::Source::Package::V2.
   * Documentation:
     - Fix grammar in dpkg-architecture(1).
       Thanks to Chris Lamb <lamby@debian.org>. Closes: #787616

scripts/Dpkg/Source/Package/V2.pm

@@ -695,6 +695,7 @@ use Dpkg::ErrorHandling;
 use Dpkg::Gettext;
 
 use File::Path qw(make_path);
+use File::Spec;
 
 sub new {
     my ($this, $dir) = @_;