autotools: fix out-of-tree building of documentation

autotools: fix out-of-tree building of documentation

When building libusb out-of-tree, ie when running configure from another
directory, building the documentation fails with:

  $ make -C doc
  make: Entering directory '/tmp/build/doc'
  make: *** No rule to make target 'libusb.png', needed by 'api-1.0'.  Stop.
  make: Leaving directory '/tmp/build/doc'

This has likely been broken by commit 4a5540a925e4 ("autotools: Fix a number of
issues"). This patch fixes that by prefixing libusb.png with @top_srcdir@/doc.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>