CMakeLists.txt: fix paths when FTDIPP is set

CMakeLists.txt: fix paths when FTDIPP is set

Use the same project name (i.e. libftdi1 and not libftdipp1) when FTDIPP
is enabled as suggested by Aurelien Jarno in
http://developer.intra2net.com/mailarchive/html/libftdi/2020/msg00044.html

Without this change, the libftdi1.pc config file defines the include
path as /usr/local/include/libftdipp1 while the ftdi.h file is actually
installed in /usr/local/include/libftdi1

This is an issue for example for libsigrok which will fail on:

In file included from src/hardware/asix-sigma/protocol.c:27:
src/hardware/asix-sigma/protocol.h:28:10: fatal error: ftdi.h: No such file or directory
   28 | #include <ftdi.h>
      |          ^~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/1427f44e36752c337791597fab47a1889552a2fe

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>