kbuild: move CONFIG_TRIM_UNUSED_KSYMS code unneeded for external module

The external module building does not need to parse this code because
KBUILD_MODULES is always set anyway.

Move this code inside the "ifeq ($(KBUILD_EXTMOD),) ... endif" block.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Nicolas Pitre <nico@linaro.org>

Makefile

diff --git a/Makefile b/Makefile
index b65cfe3..1e7f134 100644
--- a/Makefile
+++ b/Makefile
@@ -612,13 +612,6 @@
 include/config/auto.conf: ;
 endif # $(dot-config)
 
-# For the kernel to actually contain only the needed exported symbols,
-# we have to build modules as well to determine what those symbols are.
-# (this can be evaluated only once include/config/auto.conf has been included)
-ifdef CONFIG_TRIM_UNUSED_KSYMS
-  KBUILD_MODULES := 1
-endif
-
 # The all: target is the default when no target is given on the
 # command line.
 # This allow a user to issue only 'make' to build a kernel including modules
@@ -1019,6 +1012,13 @@
 	  "$(MAKE) -f $(srctree)/Makefile vmlinux"
 endif
 
+# For the kernel to actually contain only the needed exported symbols,
+# we have to build modules as well to determine what those symbols are.
+# (this can be evaluated only once include/config/auto.conf has been included)
+ifdef CONFIG_TRIM_UNUSED_KSYMS
+  KBUILD_MODULES := 1
+endif
+
 autoksyms_h := $(if $(CONFIG_TRIM_UNUSED_KSYMS), include/generated/autoksyms.h)
 
 $(autoksyms_h):