another fix for build failure found by randomconfig

diff --git a/libbb/Kbuild b/libbb/Kbuild
index 87cc698..6791299 100644
--- a/libbb/Kbuild
+++ b/libbb/Kbuild
@@ -126,5 +126,6 @@
 lib-$(CONFIG_MDEV) += xregcomp.o
 lib-$(CONFIG_LESS) += xregcomp.o
 lib-$(CONFIG_PGREP) += xregcomp.o
+lib-$(CONFIG_PKILL) += xregcomp.o
 lib-$(CONFIG_DEVFSD) += xregcomp.o
 lib-$(CONFIG_FEATURE_FIND_REGEX) += xregcomp.o