fix trivial compile errors found by randomconfig run
diff --git a/libbb/Kbuild b/libbb/Kbuild
index 5cc8d14..e562f32 100644
--- a/libbb/Kbuild
+++ b/libbb/Kbuild
@@ -120,6 +120,7 @@
 lib-$(CONFIG_AWK) += xregcomp.o
 lib-$(CONFIG_SED) += xregcomp.o
 lib-$(CONFIG_GREP) += xregcomp.o
+lib-$(CONFIG_EXPR) += xregcomp.o
 lib-$(CONFIG_MDEV) += xregcomp.o
 lib-$(CONFIG_LESS) += xregcomp.o
 lib-$(CONFIG_DEVFSD) += xregcomp.o