Make the generated oid.h dependend on the Makefile. The Makefile defines

the symbols to be extracted from the MIB and if that list of symbols is
changed, oid.h should be re-generated.

usr.sbin/bsnmpd/bsnmpd/Makefile

@@ -32,8 +32,8 @@ LDADD=	-lbegemot -lbsnmp -lwrap
 
 LDFLAGS= -export-dynamic
 
-oid.h: tree.def
-	gensnmptree -e ${XSYM} < ${.ALLSRC} > ${.TARGET}
+oid.h: tree.def Makefile
+	gensnmptree -e ${XSYM} < ${.ALLSRC:M*.def} > ${.TARGET}
 
 .ORDER: tree.c tree.h
 tree.c tree.h: tree.def