std::call_once needs -lpthread

...at least on Fedora 29:

> [LNK] Library/libsvllo.so
> ld.lld: error: undefined symbol: pthread_once
> >>> referenced by gthr-default.h:699 (/usr/lib/gcc/x86_64-redhat-linux/8/../../../../include/c++/8/x86_64-redhat-linux/bits/gthr-default.h:699)
> >>>               /data/sbergman/lo-clang2/core/workdir/CxxObject/svl/source/crypto/cryptosign.o:(__gthread_once(int*, void (*)()))

and the call to std::call_once was introduced with
d43cc00c "crashtesting: tdf#122599
NSS_CMSMessage_CreateFromDER asserts".

Change-Id: If14b8cc704dbb806c61edabb8f5621527965ceb2
Reviewed-on: https://gerrit.libreoffice.org/71118
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

svl/Library_svl.mk

@@ -111,6 +111,11 @@ $(eval $(call gb_Library_use_externals,svl,\
     nss3 \
     plc4 \
 ))
+ifeq ($(OS),LINUX)
+$(eval $(call gb_Library_add_libs,svl, \
+    -lpthread \
+))
+endif
 endif # BUILD_TYPE=DESKTOP
 endif