app-text/fbpdf: fix build failure against app-text/mupdf-1.11, bug #623468

app-text/fbpdf: fix build failure against app-text/mupdf-1.11, bug #623468

app-text/mupdf 1.11 does not provide mupdfthird library anymore
and build fails as:

  x86_64-pc-linux-gnu-gcc -o fbpdf fbpdf.o mupdf.o draw.o -Wl,-O1 -Wl,--as-needed -lmupdf -lmupdfthird -lcrypto -lm
  /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lmupdfthird
  collect2: error: ld returned 1 exit status

Now app-text/mupdf provides single 'mupdf' library.

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/623468
Package-Manager: Portage-2.3.6, Repoman-2.3.2