build: Refactor the build structure of the project

Moved all header files to `include` and all cpp files to `src`.
Updated the meson.build accordingly.

Change-Id: I9e26197b9c73b5e284cfc9d0d78a234546c282ad
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/include/meson.build b/include/meson.build
new file mode 100644
index 0000000..e4ada8f
--- /dev/null
+++ b/include/meson.build
@@ -0,0 +1 @@
+blobstore_includes = include_directories('.')