identity.hpp: add header guard
Add `#pragma once` to a header file.
Noticed this issue when trying unity build in yocto environment.
```
bbmcweblib.a.p/bmcweblib-unity0.cpp
| In file included from ../git/redfish-core/lib/network_protocol.hpp:14,
| from /home/alexander/openbmc/build/s8030/tmp/work/arm1176jzs-openbmc-linux-gnueabi/bmcweb/1.0+git/build/../git/redfish-core/src/redfish.cpp:30,
| from libbmcweblib.a.p/bmcweblib-unity0.cpp:13:
| ../git/include/identity.hpp:7:20: error: redefinition of 'std::string getHostName()'
| 7 | inline std::string getHostName()
| | ^~~~~~~~~~~
| In file included from /home/alexander/openbmc/build/s8030/tmp/work/arm1176jzs-openbmc-linux-gnueabi/bmcweb/1.0+git/build/../git/http/mutual_tls.cpp:7,
| from libbmcweblib.a.p/bmcweblib-unity0.cpp:1:
| ../git/include/identity.hpp:7:20: note: 'std::string getHostName()' previously defined here
| 7 | inline std::string getHostName()
| | ^~~~~~~~~~~
| ninja: build stopped: subcommand failed.
| INFO: autodetecting backend as ninja
```
Tested: Inspection only.
Change-Id: Ib7811ee12da763203b50fc81d39d642d2de3e212
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/include/identity.hpp b/include/identity.hpp
index 94c8f62..6810fcd 100644
--- a/include/identity.hpp
+++ b/include/identity.hpp
@@ -1,3 +1,5 @@
+#pragma once
+
#include <unistd.h>
#include <array>