powerpc: Fix typo in breakpoint kgdb code.

Currently we are getting:
 arch/powerpc/kernel/kgdb.c: In function 'kgdb_arch_exit':
 arch/powerpc/kernel/kgdb.c:492:2: error: '__debugger_breakx_match' undeclared (first use in this function)
 arch/powerpc/kernel/kgdb.c:492:2: note: each undeclared identifier is reported only once for each function it appears in

Fix the typo.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>