Fix meson warnings

As written, when we pull in the boost and boost-url subprojects, we hit
problems in the meson files.

```
../meson.build:291: WARNING: include_directories sandbox violation!
```
This commit resolves this issue, by adding explicit meson.build files
for both boost and boost-url.

Tested:
meson buildlocal

No longer returns the above error, and shows
  Subprojects
    boost                                 : YES
    boost-url                             : YES

Whereas previously those two dependencies showed up as NO.

Code compiles.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ib25340723c8cb7d6139e3e51db023e9d90e30aab
diff --git a/.gitignore b/.gitignore
index eb1140e..588f0fc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -201,6 +201,7 @@
 # subproject directories
 /subprojects/*
 !/subprojects/*.wrap
+!/subprojects/packagefiles
 
 # Meson Directories
 meson-logs