assert: fix __assert_fail()

This patch changes the definition of __assert_fail() in api/assert.h which
would allow it and other header files which include it (such as debug.hh) to
be used in mgmt submodules. Fixes conflict with declaration of
__assert_fail() in external/x64/glibc.bin/usr/include/assert.h

Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>