Disable unused-variable warning C4101 in MSVC

The C4101 warning for unused variable cannot be individually suppressed
with PG_USED_FOR_ASSERTS_ONLY, and thus cause false-positive warnings
for variables which are defined but only read/written in an assertion.
Until a satisfactory solution for per-variable suppression like how we
do for gcc and clang, disable the warning.

Discussion: https://postgr.es/m/CAJcOf-c+KniGAp31pn8TC=9a-WHXpkX-3+8-2BkaCsZchhu=8w@mail.gmail.com

Author: danielgustafsson

src/tools/msvc/Project.pm

@@ -34,7 +34,7 @@ sub _new
 		prefixincludes        => '',
 		defines               => ';',
 		solution              => $solution,
-		disablewarnings       => '4018;4244;4273;4102;4090;4267',
+		disablewarnings       => '4018;4244;4273;4101;4102;4090;4267',
 		disablelinkerwarnings => '',
 		platform              => $solution->{platform},
 	};