meson: add src/ folder

This creates a conventional structure of src/ and test/ which helps to
separate top-level files such as meson.build, OWNERS, ...
from implementation files.

Git history of individual files is still accessible via e.g.
```
git log --follow -- src/thresholds.hpp
```

Tested: code compiles.

Change-Id: Ifff8b3e70437bc6a25cd6f65afd07d8a563d1a8c
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/src/meson.build b/src/meson.build
new file mode 100644
index 0000000..43f7f96
--- /dev/null
+++ b/src/meson.build
@@ -0,0 +1,19 @@
+executable(
+    'virtual-sensor',
+    [
+        'calculate.cpp',
+        'dbusSensor.cpp',
+        'dbusUtils.cpp',
+        'main.cpp',
+        'virtualSensor.cpp',
+    ],
+    dependencies: [
+        dependency('nlohmann_json', include_type: 'system'),
+        dependency('phosphor-dbus-interfaces'),
+        dependency('phosphor-logging'),
+        dependency('sdbusplus'),
+        exprtk,
+    ],
+    install: true,
+    install_dir: get_option('libexecdir') / meson.project_name(),
+)