pcmanfm: link against libX11 to avoid build failure

Fixes:

/home/test/outputs/allpkg/host/opt/ext-toolchain/bin/../lib/gcc/arm-linux-gnueabihf/4.7.3/../../../../arm-linux-gnueabihf/bin/ld: fm-desktop.o: undefined reference to symbol 'XSendEvent'
/home/test/outputs/allpkg/host/opt/ext-toolchain/bin/../lib/gcc/arm-linux-gnueabihf/4.7.3/../../../../arm-linux-gnueabihf/bin/ld: note: 'XSendEvent' is defined in DSO /home/test/outputs/allpkg/host/usr/arm-buildroot-linux-gnueabi/sysroot/usr/lib/libX11.so.6 so try adding it to the linker command line
/home/test/outputs/allpkg/host/usr/arm-buildroot-linux-gnueabi/sysroot/usr/lib/libX11.so.6: could not read symbols: Invalid operation
collect2: error: ld returned 1 exit status
make[4]: *** [pcmanfm] Error 1

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>