package/heirloom-mailx: fix link issue with getopt

With NIOS2 toolchain, heirloom-mailx fail to build
due to missing -fPIC when linking with its own getopt.

Since static build for heirloom-mailx is disabled,
always pass -fPIC in CFLAGS.

Fixes:
http://autobuild.buildroot.net/results/333/33346f4555ed403bab7b739dd5d47814efa8eb4c



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>