x86, boot: Don't compile early_serial_console.c when !CONFIG_EARLY_PRINTK

All the code in early_serial_console.c gets compiled out if
!CONFIG_EARLY_PRINTK, but early_serial_console.o itself still gets
compiled in.  Eliminate it from the compile entirely in that case.

This does not change the generated code at all, in either case.
Signed-off-by: Josh Triplett <josh@joshtriplett.org>

arch/x86/boot/compressed/Makefile

@@ -27,9 +27,10 @@ hostprogs-y	:= mkpiggy
 HOST_EXTRACFLAGS += -I$(srctree)/tools/include
 
 vmlinux-objs-y := $(obj)/vmlinux.lds $(obj)/head_$(BITS).o $(obj)/misc.o \
-	$(obj)/string.o $(obj)/cmdline.o $(obj)/early_serial_console.o \
+	$(obj)/string.o $(obj)/cmdline.o \
 	$(obj)/piggy.o $(obj)/cpuflags.o
 
+vmlinux-objs-$(CONFIG_EARLY_PRINTK) += $(obj)/early_serial_console.o
 vmlinux-objs-$(CONFIG_RANDOMIZE_BASE) += $(obj)/aslr.o
 
 $(obj)/eboot.o: KBUILD_CFLAGS += -fshort-wchar -mno-red-zone

arch/x86/boot/compressed/early_serial_console.c

 #include "misc.h"
 
-#ifdef CONFIG_EARLY_PRINTK
-
 int early_serial_base;
 
 #include "../early_serial_console.c"
-
-#endif