Add host_error_monitor.cpp to the unit test build

Some functions are only available in host_error_monitor.cpp, so it needs
to be included in unit test builds.

Signed-off-by: Jason M. Bills <jason.m.bills@linux.intel.com>
Change-Id: Ib1e2b698fde3e3639e739a3ff3964eaba40338eb
diff --git a/src/host_error_monitor.cpp b/src/host_error_monitor.cpp
index 00177b9..ffeab44 100644
--- a/src/host_error_monitor.cpp
+++ b/src/host_error_monitor.cpp
@@ -46,7 +46,7 @@
         }
     }
 }
-static void initializeHostState()
+[[maybe_unused]] static void initializeHostState()
 {
     // Get the current host state to prepare to start the signal monitors
     conn->async_method_call(
@@ -72,7 +72,8 @@
         "xyz.openbmc_project.State.Host", "CurrentHostState");
 }
 
-static std::shared_ptr<sdbusplus::bus::match_t> startHostStateMonitor()
+[[maybe_unused]] static std::shared_ptr<sdbusplus::bus::match_t>
+    startHostStateMonitor()
 {
     return std::make_shared<sdbusplus::bus::match_t>(
         *conn,
@@ -119,6 +120,7 @@
 }
 } // namespace host_error_monitor
 
+#ifndef UNIT_TEST
 int main(int /*argc*/, char* /*argv*/[])
 {
     // setup connection to dbus
@@ -142,3 +144,4 @@
 
     return 0;
 }
+#endif
diff --git a/tests/meson.build b/tests/meson.build
index 545c988..2cadb5c 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -29,6 +29,8 @@
             executable(
                 unit_test,
                 unit_test + '.cpp',
+                '../src/host_error_monitor.cpp',
+                cpp_args: '-DUNIT_TEST',
                 include_directories: incs,
                 dependencies: deps
                 + [