qt-mobility: fix build without X11

qt-mobility: fix build without X11

else we get :
arm-oe-linux-gnueabi-g++  -march=armv7-a -mthumb-interwork -mfloat-abi=softfp -mfpu=neon .../...
ibQtSystemInfoE.so.1 -o libQtSystemInfoE.so.1.2.0 .../... -lblkid -ludev -lX11 -lXrandr -lQtDBusE -lQtXmlE -lQtGuiE -lEGL -lQtNetworkE -lQtCoreE -lpthread
.../build/tmp-defaultsetup-eglibc-eglibc/sysroots/x86_64-linux/usr/libexec/armv7a-vfp-neon-oe-linux-gnueabi/gcc/arm-oe-linux-gnueabi/4.8.1/ld: cannot find -lXrandr
make[2]: *** [../../lib/libQtSystemInfoE.so.1.2.0] Error 1

(From OE-Core rev: 21857f0842501b0c72c77e5fb290aec85b1c28ff)

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>