IWYU: std::vector requires including <vector> in OnDeviceHeadModel

Fix build error:
../../components/omnibox/browser/on_device_head_model.h:68:15: error: ‘vector’ in namespace ‘std’ does not name a template type

Bug: 957519
Change-Id: Idad1853af6afbf31faf288b6b5a73007497338a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453452Reviewed-by: Angela Yoeurng <yoangela@chromium.org>
Commit-Queue: José Dapena Paz <jdapena@igalia.com>
Cr-Commit-Position: refs/heads/master@{#814796}

components/omnibox/browser/on_device_head_model.h

@@ -7,6 +7,7 @@
 
 #include <string>
 #include <utility>
+#include <vector>
 
 // On device head suggest feature uses an on device model which encodes some
 // top queries into a radix tree (https://en.wikipedia.org/wiki/Radix_tree), to