dpkg-source: tighten the regexp used to ignore the .pc directory of quilt

The "." was not escaped and the regex incorrectly matched an "ipc"
directory for example.

Reported-by: Mike Hommey <glandium@debian.org>

debian/changelog

@@ -8,6 +8,8 @@ dpkg (1.16.1) UNRELEASED; urgency=low
   * Let update-alternatives fsync() its administrative files before
     moving them in place to avoid empty files with some filesystems.
     LP: #344019
+  * Tighten the regexp used by dpkg-source to ignore the .pc directory of
+    quilt. Thanks to Mike Hommey for noticing the problem.
 
   [ Guillem Jover ]
   * Install deb-src-control(5) man pages in dpkg-dev. Closes: #620520

scripts/Dpkg/Source/Package/V3/quilt.pm

@@ -262,7 +262,7 @@ sub prepare_build {
     # stamp file created by ourselves
     my $func = sub {
         return 1 if $_[0] =~ m{^debian/patches/series$} and -l $_[0];
-        return 1 if $_[0] =~ /^.pc(\/|$)/;
+        return 1 if $_[0] =~ /^\.pc(\/|$)/;
         return 1 if $_[0] =~ /$self->{'options'}{'diff_ignore_regexp'}/;
         return 0;
     };