package/exim: use libnsl only with glibc

Only (e)glibc provides libnsl, uclibc provides only a stub, and musl
doesn't implement it at all.

Fixes compilation using this defconfig

    BR2_arm=y
    BR2_cortex_a7=y
    BR2_STATIC_LIBS=y
    BR2_TOOLCHAIN_EXTERNAL=y
    BR2_PACKAGE_OPENSSL=y
    BR2_PACKAGE_EXIM=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>