package/pistache: fix static build

package/pistache: fix static build

Don't build shared library with BR2_SHARED_LIBS=OFF to avoid the
following build failure:

[100%] Linking CXX shared library libpistache.so
make[3]: Leaving directory `/home/buildroot/autobuild/run/instance-1/output-1/build/pistache-f2f5a50fbfb5b8ef6cf1d3d2a9d442a8270e375d'
[100%] Built target pistache_static
/home/buildroot/autobuild/run/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-musl/8.3.0/../../../../x86_64-buildroot-linux-musl/bin/ld: /home/buildroot/autobuild/run/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-musl/8.3.0/crtbeginT.o: relocation R_X86_64_32 against hidden symbol `__TMC_END__' can not be used when making a shared object

Fixes:
 - http://autobuild.buildroot.org/results/6cc/6cc76b3e76defa9b8154568a6ebcd6bf4cadd334/build-end.log

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: update upstream status now they merged it]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>