elf.h: add STN_UNDEF

ELF uses index STN_UNDEF = 0 for undefined symbols.  glibc and musl
define this constant in elf.h, and it's needed to build the kernel's
vDSO selftest suite.

Test: Overwrite elf.h in standalone NDK toolchain, then
	cd $kernel/tools/testing/selftests/vDSO && \
	make CC="aarch64-linux-android-gcc -pie" vdso_test
Change-Id: I495586e9ca43d2f0d148ded9f4bd61ec8d14d53b
Signed-off-by: Greg Hackmann <ghackmann@google.com>