Make our perfect hash functions be valid C++.

While C is happy to cast "const void *" to "const unsigned char *"
silently, C++ insists on an explicit cast.  Since we put these
functions into header files, cpluspluscheck whines about that.
Add the cast to pacify it.

Discussion: https://postgr.es/m/b517ec3918d645eb950505eac8dd434e@gaz-is.ru

src/tools/PerfectHash.pm

@@ -131,7 +131,7 @@ sub generate_hash_function
 	}
 	$f .= sprintf "\n" if ($nhash % 8 != 0);
 	$f .= sprintf "\t};\n\n";
-	$f .= sprintf "\tconst unsigned char *k = key;\n";
+	$f .= sprintf "\tconst unsigned char *k = (const unsigned char *) key;\n";
 	$f .= sprintf "\tsize_t\t\tkeylen = %d;\n", $options{fixed_key_length}
 	  if (defined $options{fixed_key_length});
 	$f .= sprintf "\tuint32\t\ta = %d;\n",   $hash_seed1;