kbuild: modpost: handle KBUILD_EXTRA_SYMBOLS only for external modules

KBUILD_EXTRA_SYMBOLS makes sense only when building external modules.
Moreover, the modpost sets 'external_module' if the -e option is given.

I replaced $(patsubst %, -e %,...) with simpler $(addprefix -e,...)
while I was here.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>