package/gkrellm: fix client build

package/gkrellm: fix client build

Set X11_LIBS to avoid the following build failure:

/home/buildroot/autobuild/run/instance-1/output-1/host/bin/aarch64-linux-gnu-gcc main.o alerts.o battery.o base64.o clock.o cpu.o disk.o fs.o hostname.o inet.o mail.o mem.o net.o proc.o sensors.o uptime.o chart.o panel.o config.o gui.o krell.o plugins.o pixops.o client.o utils.o sysdeps-unix.o deprecated.o log.o winops-x11.o  -o gkrellm \
		 -L/home/buildroot/autobuild/run/instance-1/output-1/host/bin/../aarch64-buildroot-linux-gnu/sysroot/usr/lib -lgtk-x11-2.0 -lgdk-x11-2.0 -lpangocairo-1.0 -latk-1.0 -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lpangoft2-1.0 -lpango-1.0 -lgobject-2.0 -lharfbuzz -lfontconfig -lfreetype -Wl,--export-dynamic -lgmodule-2.0 -lglib-2.0 -lgthread-2.0 -pthread -lglib-2.0   -L/usr/X11R6/lib -lX11 -lSM -lICE  -L/home/buildroot/autobuild/run/instance-1/output-1/host/bin/../aarch64-buildroot-linux-gnu/sysroot/usr/lib -lssl -lcrypto    -lm -Wl,-E
aarch64-linux-gnu-gcc: ERROR: unsafe header/library path used in cross-compilation: '-L/usr/X11R6/lib'

Fixes:
 - http://autobuild.buildroot.org/results/fff9a48efe3818f67a8f4b0fe3a3a605e4985b3b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>