package/linux-pam: find libxcrypt through pkg-config

Find libxcryt through pkg-config to avoid the following build failure:

/home/buildroot/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/riscv64-buildroot-linux-musl/10.2.0/../../../../riscv64-buildroot-linux-musl/bin/ld: .libs/passverify.o: in function `.L30':
passverify.c:(.text+0x368): undefined reference to `crypt_checksalt'

Fixes:
 - http://autobuild.buildroot.org/results/20b14e222b35c2d1269960075832b784ba81aa1a



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>