package/libftdi: fix libftdi.pc

This patch fixes libftdi pkg-config module, so that packages linking
against libftdi correctly have all needed CFLAGS and LIBS defined,
especially when running a static build.

Fixes:
  http://autobuild.buildroot.org/results/e90/e90b4d5ad79d99487f21c9d18581e8eba7034501/



Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>