move utmp.h include to libbb.h

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/include/libbb.h b/include/libbb.h
index 32e25a8..182b479 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -50,6 +50,9 @@
 # include <selinux/flask.h>
 # include <selinux/av_permissions.h>
 #endif
+#if ENABLE_FEATURE_UTMP
+# include <utmp.h>
+#endif
 #if ENABLE_LOCALE_SUPPORT
 # include <locale.h>
 #else