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/server/IptablesRestoreController.cpp b/server/IptablesRestoreController.cpp
index 88d88f6..eb67b75 100644
--- a/server/IptablesRestoreController.cpp
+++ b/server/IptablesRestoreController.cpp
@@ -347,7 +347,7 @@
 
 int IptablesRestoreController::execute(const IptablesTarget target, const std::string& command,
                                        std::string *output) {
-    std::lock_guard<std::mutex> lock(mLock);
+    std::lock_guard lock(mLock);
 
     std::string buffer;
     if (output == nullptr) {