Add exceptions for directories with currently unknown licenses

It is an ongoing process (cf. #165) to investigate license status
and add it to each file.

.reuse/dep5

@@ -10,6 +10,27 @@ Files:
 Copyright: 2011-2020 The SV-Benchmarks community
 License: CC0-1.0
 
+Files:
+ clauses
+ c/bitvector
+ c/ddv-machzwd
+ c/ldv-memsafety
+ c/ldv-regression
+ c/ldv-README.txt
+ c/list-properties
+ c/locks
+ c/loops
+ c/ntdrivers
+ c/ntdrivers-simplified
+ c/pthread
+ c/pthread-ext
+ c/seq-pthread
+ c/systemc
+ java/rtems-lock-model
+ _
+Copyright: unknown
+License: LicenseRef-unknown
+
 Files:
  c/Juliet_Test
  c/array-cav19

LICENSES/LicenseRef-unknown.txt

@@ -0,0 +1,3 @@
+The license of these files are currently unknown.
+Please contribute to https://github.com/sosy-lab/sv-benchmarks/issues/165
+by providing information about licenses if you know more.