libiscsi: only build the test tool and ld-iscsi if we have shared libs

Backporting an upstream patch to fix a failure when doing a static
build:

/br/output/host/usr/bin/mipsel-ctng-linux-uclibc-gcc -shared -o
ld_iscsi.so ld_iscsi.o -ldl
/br/output/host/opt/ext-toolchain/bin/../lib/gcc/mipsel-ctng-linux-uclibc/4.8.2/../../../../mipsel-ctng-linux-uclibc/bin/ld:
ld_iscsi.o: relocation R_MIPS_HI16 against `__gnu_local_gp' can not be
used when making a shared object; recompile with -fPIC
ld_iscsi.o: could not read symbols: Bad value
collect2: error: ld returned 1 exit status

Upstream commit:
  https://github.com/sahlberg/libiscsi/commit/3d6c2be3428d88682cecfa2a4ad172aaacffb318

Fixes:
  http://autobuild.buildroot.net/results/7a9/7a9caf1f4080c2c4b04ee3b13c1240f475a22ea7/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>