powerpc: io-workarounds.c was implicitly getting init_mm

It was coming in via device.h --> module.h etc. but we want to
clean that up.  So explicitly include the header where init_mm
is being declared.
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

arch/powerpc/kernel/io-workarounds.c

@@ -12,6 +12,7 @@
 #undef DEBUG
 
 #include <linux/kernel.h>
+#include <linux/sched.h>	/* for init_mm */
 
 #include <asm/io.h>
 #include <asm/machdep.h>