meson: sizes.h: add includedir to install path

The install path for a generated `sizes.h` is missing includedir.
This results in the header being installed into `/usr/libpldm/sizes.h`
rather than `/usr/include/libpldm/sizes.h`.  This was detected by a
recipe failure in the Yocto bump for this repository.

Tested:
```
$ meson install -C builddir --destdir /tmp/libpldm
$ find /tmp/libpldm -name sizes.h
/tmp/libpldm/usr/local/include/libpldm/sizes.h
```

Fixes: 8c2bfb141d11 ("libpldm: Add firmware update FD responder")
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I7dd8cccc7c51302f87005ec540c523ab4561b8f4
diff --git a/CHANGELOG.md b/CHANGELOG.md
index ab359ea..2f7f8da 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -733,3 +733,4 @@
 3. transport: Match on response in pldm_transport_send_recv_msg()
 4. requester: Add check before accessing hdr in pldm_recv()
 5. bios_table: pldm_bios_table_attr_entry_string_info_check() NULL deref
+6. meson: sizes.h: add includedir to install path
diff --git a/include/libpldm/meson.build b/include/libpldm/meson.build
index 2887f0b..3a8c708 100644
--- a/include/libpldm/meson.build
+++ b/include/libpldm/meson.build
@@ -61,5 +61,5 @@
     input: 'sizes.h.in',
     output: 'sizes.h',
     install: true,
-    install_dir: 'libpldm',
+    install_dir: get_option('includedir') / 'libpldm',
 )