build: fix linking libqemutestdriver with LTO enabled

openSUSE Factory is in the process of enabling Link Time Optimization [0]
and stumbled upon missing symbols when linking libqemutestdriver

libtool: link: gcc -shared  -fPIC -DPIC  ../src/libvirt_qemu_probes.o  \
-Wl,--whole-archive ../src/.libs/libvirt_driver_qemu_impl.a \
../src/.libs/libvirt_driver_network_impl.a \
../src/.libs/libvirt_driver_storage_impl.a -Wl,--no-whole-archive  \
-lcap-ng -lgnutls -lnl-route-3 -lnl-3 -ldbus-1 -lselinux -lapparmor \
/usr/lib/libxml2.so -ldl -lz -llzma -lm -lblkid -ltirpc  \
-fstack-protector-strong -O2 -fstack-protector-strong -flto=16 -g -O2 \
-fstack-protector-strong -flto=16 -g \
-Wl,-soname -Wl,libqemutestdriver.so -o .libs/libqemutestdriver.so
...
/usr/lib/gcc/i586-suse-linux/9/../../../../i586-suse-linux/bin/ld: ./.libs/libqemutestdriver.so: undefined reference to `mdir_name'
/usr/lib/gcc/i586-suse-linux/9/../../../../i586-suse-linux/bin/ld: ./.libs/libqemutestdriver.so: undefined reference to `rpl_ioctl'
/usr/lib/gcc/i586-suse-linux/9/../../../../i586-suse-linux/bin/ld: ./.libs/libqemutestdriver.so: undefined reference to `rpl_pipe2'
/usr/lib/gcc/i586-suse-linux/9/../../../../i586-suse-linux/bin/ld: ./.libs/libqemutestdriver.so: undefined reference to `recvfd'
/usr/lib/gcc/i586-suse-linux/9/../../../../i586-suse-linux/bin/ld: ./.libs/libqemutestdriver.so: undefined reference to `c_strcasecmp'

The missing symbols are provided by GNUlib. Add it when linking
libqemutestdriver.

[0] https://en.opensuse.org/openSUSE:LTOSigned-off-by: NJim Fehlig <jfehlig@suse.com>
Acked-by: NMichal Privoznik <mprivozn@redhat.com>

tests/Makefile.am

@@ -557,7 +557,7 @@ endif WITH_DTRACE_PROBES
 
 libqemutestdriver_la_SOURCES =
 libqemutestdriver_la_LDFLAGS = $(DRIVERLIB_LDFLAGS)
-libqemutestdriver_la_LIBADD = $(qemu_LDADDS)
+libqemutestdriver_la_LIBADD = $(qemu_LDADDS) $(LDADDS)
 
 qemucpumock_la_SOURCES = \
 	qemucpumock.c testutilshostcpus.h