powerpc: Fix build warning on 32-bit PPC

I am getting the following warning when I build kernel 4.9-git on my
PowerBook G4 with a 32-bit PPC processor:

    AS      arch/powerpc/kernel/misc_32.o
  arch/powerpc/kernel/misc_32.S:299:7: warning: "CONFIG_FSL_BOOKE" is not defined [-Wundef]

This problem is evident after commit 989cea5 ("kbuild: prevent
lib-ksyms.o rebuilds"); however, this change in kbuild only exposes an
error that has been in the code since 2005 when this source file was
created.  That was with commit 9994a33 ("powerpc: Introduce
entry_{32,64}.S, misc_{32,64}.S, systbl.S").

The offending line does not make a lot of sense.  This error does not
seem to cause any errors in the executable, thus I am not recommending
that it be applied to any stable versions.

Thanks to Nicholas Piggin for suggesting this solution.

Fixes: 9994a33 ("powerpc: Introduce entry_{32,64}.S, misc_{32,64}.S, systbl.S")
Signed-off-by: Larry Finger <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>

arch/powerpc/kernel/misc_32.S

@@ -296,7 +296,7 @@ _GLOBAL(flush_instruction_cache)
 	lis	r3, KERNELBASE@h
 	iccci	0,r3
 #endif
-#elif CONFIG_FSL_BOOKE
+#elif defined(CONFIG_FSL_BOOKE)
 BEGIN_FTR_SECTION
 	mfspr   r3,SPRN_L1CSR0
 	ori     r3,r3,L1CSR0_CFI|L1CSR0_CLFC