package/pugixml: unconditionally set BUILD_DEFINES

The ifdef construct intended to avoid passing -DBUILD_DEFINES="" was
never needed, because even upstream version v1.9 used in 2019 when the
configuration options were added, can handle an empty string for
BUILD_DEFINES.
In fact an empty string is the default for v1.9 if it is not passed
during configure.

Also, the host variant already sets BUILD_DEFINES unconditionally.

So remove the unneeded conditional.

Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Theo Debrouwere <t.debrouwere@televic.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>