package/olsr: needs -fPIC to avoid build failure

When building on Arc, Microblaze, Riscv32, Riscv64, package olsr needs
to be compiled with -fPIC, so append -fPIC to CFLAGS when invoking make
by default, since it seems to be mandatory in general and it's harmless
for other architectures.

Fixes:
http://autobuild.buildroot.net/results/9d6/9d6d69133789aba16daeb6dc53721597e6be9806/
http://autobuild.buildroot.net/results/b85/b85d27b3a4bab5d68460401ff49e624177d55afa/
http://autobuild.buildroot.net/results/0d3/0d3489b14f4f4f0d4837d00209b5c1ee8d6f6a98/
http://autobuild.buildroot.net/results/d9a/d9a143c8e9a175741acfafd17b63544d04744ac9/



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>