libxslt: fix host compilation

Do not build against libgcrypt on the host, since we don't build
libgcrypt ourself, and it might fail even when libgcrypt is installed
on the host:

...configure...
checking for libgcrypt-config... /usr/bin/libgcrypt-config
Crypto extensions will be available.

...build...
gcc: /usr/lib/libgcrypt.so: No such file or directory

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>