genhtml: Fix apply_prefix regex on Windows (#442)

Ensure that `$lcovutil::dirseparator`, which can be a backslash, is properly escaped in the regex. Fixes an error of the form:
```
genhtml: ERROR: Unmatched ) in regex; marked by <-- HERE in m/^C\:\\b\\absmfft5\\execroot\\_main\\_tmp\\841d8b670c55601362e55b1ef828d048\\workspace\(.*) <-- HERE $/
```

Signed-off-by: Fabian Meumertzheim <fabian@meumertzhe.im>

Author: fmeum

bin/genhtml

@@ -13806,7 +13806,7 @@ sub apply_prefix($@)
             return 'root';
         }
         if ($prefix ne "" &&
-            $filename =~ /^\Q$prefix\E$lcovutil::dirseparator(.*)$/) {
+            $filename =~ /^\Q$prefix$lcovutil::dirseparator\E(.*)$/) {
             return substr($filename, length($prefix) + 1);
         }
     }