libnss-nis: Do not export _nss_nis_getsecretkey when libc does not have xdecrypt

libnss-nis: Do not export _nss_nis_getsecretkey when libc does not have xdecrypt

Fixes build errors ( seen with lld particularly )

| x86_64-yoesdk-linux-ld.lld: error: version script assignment of 'NSS_NIS_PRIVATE_2.0' to symbol '_nss_nis_getsecretkey' failed: symbol not defined
| x86_64-yoesdk-linux-clang: error: linker command failed with exit code 1 (use -v to see invocation)
| make[3]: *** [Makefile:642: libnss_nis.la] Error 1
| make[3]: Leaving directory '/mnt/b/yoe/master/build/tmp/work/x86_64-nativesdk-yoesdk-linux/nativesdk-libnss-nis/3.2/build/src'

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>