Remove non-existing variable reference in MSVC's Solution.pm

The version string is grabbed from PACKAGE_VERSION in pg_config.h in the
MSVC build since 8f4fb4c6, but an error message referenced a variable
that existed before that.  This had no consequences except if one messes
up enough with the version number of the build.

Author: Anton Voloshin
Discussion: https://postgr.es/m/af79ee1b-9962-b299-98e1-f90a289e19e6@postgrespro.ru
Backpatch-through: 13

src/tools/msvc/Solution.pm

@@ -176,7 +176,7 @@ sub GenerateFiles
 
 			if ($package_version !~ /^(\d+)(?:\.(\d+))?/)
 			{
-				confess "Bad format of version: $self->{strver}\n";
+				confess "Bad format of version: $package_version\n";
 			}
 			$majorver = sprintf("%d", $1);
 			$minorver = sprintf("%d", $2 ? $2 : 0);