roots: Remove #include "common.h".

And add the missing include of <string.h> (e.g. for strcmp(3)).

Minor update to the arg of fs_mgr_get_entry_for_mount_point(), which now
accepts std::string.

Test: mmma bootable/recovery
Change-Id: I9cb8c31fe71b5a053f4d84bf1aba00e96c02ed03