Summary:
Ubuntu 18.04 (and presumably other Debian-based distributions) install
dwarf.h at /usr/include/libdwarf/dwarf.h.  However other platforms like Mac
typically install dwarf.h directly inside include/ without a libdwarf
subdirectory.

Commit e66fb34c previously changed the code to look only for "dwarf.h" to
support Homebrew installations of dwarfutils on Mac, but accidentally broke
finding dwarf.h on Ubuntu in the process.  This change should hopefully work
on both platforms.

Fixes: #905

Reviewed By: yfeldblum

Differential Revision: D9212428

fbshipit-source-id: a4520a47718a2da25e597ac7350344931ba21634