flare-engine: fix debug build

If BR2_ENABLE_DEBUG is set, use RelWithDebInfo instead of default Debug
as Debug will add -pg (gprof) which is not always available on toolchain

Fixes:
 - http://autobuild.buildroot.org/results/a12ae622a44bbe025c3a8b7e8e4c253b52927ae8



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>