vnc: Fix compilation with --enable-vnc-png

Commit f26e428da505709ec03b2ed2c9eb3db82b30bd7b fixed compilation
with --enable-vnc-png, but broke it with --enable-vnc-png.

The breakage is caused by pngconfig.h which checks whether
setjmp.h was already included and fails because qemu-common.h
includes setjmp.h.

The check is disabled by defining PNG_SKIP_SETJMP_CHECK.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
