ppc4xx: Fix compilation error on ML2 board

ppc4xx: Fix compilation error on ML2 board

Recently this compilation error occurs:

Configuring for ML2 board...
traps.c: In function 'MachineCheckException':
traps.c:159: error: 'debugger_exception_handler' undeclared (first use
	in this function)
traps.c:159: error: (Each undeclared identifier is reported only once
traps.c:159: error: for each function it appears in.)

This patch now fixes it by including kgdb.h

Signed-off-by: Stefan Roese <sr@denx.de>