package/sofia-sip: correct passing of '--enable-ndebug'

package/sofia-sip: correct passing of '--enable-ndebug'

The preprocessor option NDEBUG, triggered by the configure option
'--enable-ndebug', should be read as 'no-debug'. When NDEBUG is set, asserts
are _disabled_.

The sofia-sip package had inverted logic, and set '--enable-ndebug' when
BR2_ENABLE_DEBUG was enabled, while it should be the other way around.

Reported-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>