powerpc: Correctly disable latent entropy GCC plugin on prom_init.o

Commit 38addce8 ("gcc-plugins: Add latent_entropy plugin") excludes
certain powerpc early boot code from the latent entropy plugin by adding
appropriate CFLAGS. It looks like this was supposed to cover
prom_init.o, but ended up saying init.o (which doesn't exist) instead.
Fix the typo.

Fixes: 38addce8 ("gcc-plugins: Add latent_entropy plugin")
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/Makefile

@@ -15,7 +15,7 @@ CFLAGS_btext.o		+= -fPIC
 endif
 
 CFLAGS_cputable.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
-CFLAGS_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
+CFLAGS_prom_init.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 CFLAGS_btext.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)