Update git submodules

* Update docs/submodules/policy/engine.git from branch 'master'
  to 51d9bb17422ae08042971e9cb87bd6a2576a5cdb
  - Unit/SONAR/Checkstyle in ONAP-REST

    Util package of ONAP-REST, with JUnit added and
    SONAR/Checkstyle issues addressed. Coverage increased from 51% to 73% in
    large PolicyValidation class.

    Issue-ID: POLICY-2131
    Change-Id: Iffa90e2dfa155d3e9cac114032fb9ca0df2cbab0
    Signed-off-by: liamfallon <liam.fallon@est.tech>

diff --git a/docs/submodules/policy/engine.git b/docs/submodules/policy/engine.git
index 58c44ae..51d9bb1 160000 (submodule)
--- a/docs/submodules/policy/engine.git
+++ b/docs/submodules/policy/engine.git
@@ -1 +1 @@
-Subproject commit 58c44ae81489deca05f57812c19f25c3897fe57a
+Subproject commit 51d9bb17422ae08042971e9cb87bd6a2576a5cdb