Makefile.extrawarn: remove -Wnested-externs warning

The -Wnested-externs warning has become useless with gcc, since
this warns every time that BUILD_BUG_ON() or similar macros
are used.

With clang, the warning option does nothing to start with, so
just remove it entirely.
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.extrawarn

@@ -61,7 +61,6 @@ endif
 ifneq ($(findstring 2, $(KBUILD_EXTRA_WARN)),)
 
 KBUILD_CFLAGS += -Wdisabled-optimization
-KBUILD_CFLAGS += -Wnested-externs
 KBUILD_CFLAGS += -Wshadow
 KBUILD_CFLAGS += $(call cc-option, -Wlogical-op)
 KBUILD_CFLAGS += -Wmissing-field-initializers