avoid trying to gcov unincluded files

some C files included with Imager are not included in every build
avoid the noise of trying to gcov them when they aren't

imcover.perl

@@ -41,6 +41,8 @@
 my %paths;
 for my $filename (keys %$mani) {
   next unless $filename =~ /\.(xs|c|im)$/;
+  (my $gcda = $filename) =~ s/\.\w+$/.gcda/;
+  next unless -f $gcda;
   if ($filename =~ m!^(\w+)/(\w+\.\w+)$!) {
     push @{$paths{$1}}, $2;
   }