pcmanfm: replace dependency on X.org server by dependency on libX11

There is no reason for a client program like pcmanfm to depend on the
X.org server. Instead, it should depend on the appropriate X
client libraries, in this case libX11.

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