Remove seemingly unneeded include directory in MSVC scripts

This appears to have been added way back in ee3b418 but it's a little
unclear why the change made in that commit is even needed given that
320c7eb, dated 18 months earlier, added code to copy fmgroids.h to
src/include/utils.

amcheck seems to get away without adding the additional include directory,
so perhaps dblink can get away with it too.

This builds ok in my VS2017 environment, but the buildfarm may serve as a
reminder about why ee3b418 was required.  There's only one way to find
out for sure.

Discussion: https://postgr.es/m/CAApHDvpo6g5csCTjc_0C7DMvgFPomVb0Rh-AcW5afd=Ya=LRuw@mail.gmail.com

Author: david-rowley

src/tools/msvc/Mkvcbuild.pm

@@ -41,7 +41,7 @@ my @contrib_uselibpq =
 my @contrib_uselibpgport   = ('libpq_pipeline', 'oid2name', 'vacuumlo');
 my @contrib_uselibpgcommon = ('libpq_pipeline', 'oid2name', 'vacuumlo');
 my $contrib_extralibs     = { 'libpq_pipeline' => ['ws2_32.lib'] };
-my $contrib_extraincludes = { 'dblink'         => ['src/backend'] };
+my $contrib_extraincludes = {};
 my $contrib_extrasource   = {
 	'cube' => [ 'contrib/cube/cubescan.l', 'contrib/cube/cubeparse.y' ],
 	'seg'  => [ 'contrib/seg/segscan.l',   'contrib/seg/segparse.y' ],