-pthread flag missing

[vanyasem]

Building bionic branch on Arch GNU/Linux

[ 62%] Linking CXX executable QtEventFeederTest
/usr/bin/ld: ../../libGMock.a(gmock-all.cc.o): undefined reference to symbol 'pthread_key_delete@@GLIBC_2.2.5'
/usr/lib/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[2]: *** [tests/mirserver/QtEventFeeder/CMakeFiles/QtEventFeederTest.dir/build.make:132: tests/mirserver/QtEventFeeder/QtEventFeederTest] Error 1

-pthread is missing in target_link_libraries in tests/mirserver/QtEventFeeder/CMakeLists.txt

Same as ubports/url-dispatcher#4