Fixed compilation of the boot loader on GCC 2.

Not sure why but GCC 2 seems to be ignoring the extern "C" in smp.h when
compiling smp_boot_other_cpus() resulting in undefined references. Fixed
by putting extern "C" on the definition too.

src/system/boot/platform/bios_ia32/smp.cpp

@@ -420,7 +420,7 @@ smp_init_other_cpus(void)
 }
 
 
-void
+extern "C" void
 smp_boot_other_cpus(void (*entryFunc)())
 {
 	if (gKernelArgs.num_cpus < 2)