Let lock_guard deduce its template argument

No functional change, this is a cleanup.

With C++17, it's no longer necessary to specify the teplate argument
when it can be deduced from the types of constructor arguments. This
allows de-cluttering our locking statements.

To avoid typos, this patch was mechanically generated:

  perl -p -i -e 's/std::lock_guard<std::mutex>/std::lock_guard/g' \
    $(find . -name '*.cpp' -o -name '*.h')

Change-Id: Ibb15d9a6c5b1c861d81353e47d25474eb1d4c2df
diff --git a/libnetdutils/Log.cpp b/libnetdutils/Log.cpp
index 98f2d13..07d5952 100644
--- a/libnetdutils/Log.cpp
+++ b/libnetdutils/Log.cpp
@@ -202,7 +202,7 @@
             break;
     }
 
-    std::lock_guard<std::shared_mutex> guard(mLock);
+    std::lock_guard guard(mLock);
     mEntries.push_back(makeTimestampedEntry(entry));
     while (mEntries.size() > mMaxEntries) mEntries.pop_front();
 }