[klibc] Need EXTRA_KLIBCCFLAGS for O= to work correctly.

For the syscalls build, need EXTRA_KLIBCCFLAGS in order for the build
not to break with O=.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/usr/klibc/syscalls/Kbuild b/usr/klibc/syscalls/Kbuild
index debcd16..e7ae1d2 100644
--- a/usr/klibc/syscalls/Kbuild
+++ b/usr/klibc/syscalls/Kbuild
@@ -28,6 +28,8 @@
 # All the syscall stubs
 clean-files += *.o *.S *.c *.list *.bin
 
+EXTRA_KLIBCCFLAGS := -I$(srctree)/$(src)
+
 quiet_cmd_makelist = LIST    $@
       cmd_makelist = echo '$(filter-out FORCE,$^)' > $@