Add missing pthread library for server

Add missing pthread library for server

While building snapper with Boost 1.67, the following error
occurred,

/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: Client.o: undefined reference to symbol 'pthread_condattr_setclock@@GLIBC_2.3.3'
/lib64/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[2]: *** [Makefile:440: snapperd] Error 1
make[2]: Leaving directory '/home/abuild/rpmbuild/BUILD/snapper-0.5.4/server'