Some versions of png.h cannot be included after setjmp.h,
even when PNG_SKIP_SETJMP_CHECK was defined.

setjmp.h was included from qemu-common.h and is not needed there.
Removing the include statement fixes compilation of ui/vnc-enc-tight.c
with CONFIG_VNC_PNG defined.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>