package/linux-pam: add libxcrypt optional dependency

Add libxcrypt optional dependency and fix the following build failure
with libxcrypt and uclibc-ng raised since the addition of libxcrypt in
commit 464bbe26:

/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabihf/9.3.0/../../../../arm-buildroot-linux-uclibcgnueabihf/bin/ld: unix_chkpwd-passverify.o: in function `verify_pwd_hash':
passverify.c:(.text+0xab4): undefined reference to `crypt_checksalt'

Fixes:
 - http://autobuild.buildroot.org/results/65d68b7c9c7de1c7cb0f941ff9982f93a49a56f8



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>