Add dependency to allow ndpi library users to build

When trying to build an application using ndpi I get the following:

[   39s] /usr/bin/ld: cannot find -lgcrypt
[   39s] collect2: error: ld returned 1 exit status

it includes the following link options, which came from pkg-config

... /usr/lib/x86_64-linux-gnu/libndpi.so -lgcrypt ...

$ cat /usr/lib/x86_64-linux-gnu/pkgconfig/libndpi.pc
prefix=/usr
exec_prefix=${prefix}
libdir=${prefix}/lib/x86_64-linux-gnu
includedir=${prefix}/include

Name: libndpi
Description: deep packet inspection library
Version: 3.4.0
Libs: -L${libdir} -lndpi  -lgcrypt
Cflags: -I${includedir}/ndpi

All package dependencies of libndpi-dev are installed:

abuild@Precision-7520:~$ dpkg -l | grep gcrypt
ii  libgcrypt20:amd64                       1.8.4-5                          amd64        LGPL Crypto library - runtime library
abuild@Precision-7520:~$ dpkg -l | grep ndpi
ii  libndpi-dev                             3.4-1                            amd64        extensible deep packet inspection library - development files
ii  libndpi3.4:amd64                        3.4-1                            amd64        extensible deep packet inspection library - shared library