heimdal: Do not build a redundant source file

Heimdal's lib/hdb/db3.c is only built if DB3 is enabled, i.e. #if HAVE_DB3.
FreeBSD's bdb is DB1. Therefore the entire db3.c file is #ifdef'd out.
Let's avoid building a file that results in a useless object file.

(cherry picked from commit 7ee18f5aad)

kerberos5/lib/libhdb/Makefile

@@ -39,7 +39,6 @@ INCS=	hdb-protos.h \
 
 SRCS=	common.c \
 	db.c \
-	db3.c \
 	dbinfo.c \
 	ext.c \
 	hdb-ldap.c \