Add subproject directory & wrapper files

This commit enables the isolated build of this repository with meson.
This contribution was made possible by Patrick's [blog post][1].

Building standalone with `meson setup builddir && ninja -C builddir`
worked up to the point when `phosphor-logging` raised an
[error][2]. So I cannot state that it is fully tested, but setting up
the `builddir` correctly should've been half the way.

[1]: http://www.stwcx.xyz/blog/2021/04/18/meson-subprojects.html
[2]: https://github.com/openbmc/phosphor-logging/issues/22

Signed-off-by: Patrik Tesarik <patrik.tesarik@rub.de>
Change-Id: I8ce6c9292edbb6d72414edd8794c337f30bc0734
diff --git a/subprojects/phosphor-logging.wrap b/subprojects/phosphor-logging.wrap
new file mode 100644
index 0000000..a039fcf
--- /dev/null
+++ b/subprojects/phosphor-logging.wrap
@@ -0,0 +1,3 @@
+[wrap-git]
+url = https://github.com/openbmc/phosphor-logging.git
+revision = HEAD