build: add $(BSON_LIBS) for mongoc-rpc-validate test program.

This should fix builds on systems like Ubuntu with non-standard GNU ld
configurations.

Author: chergert

examples/Makefile.am

@@ -38,6 +38,9 @@ noinst_PROGRAMS += mongoc-rpc-validate
 mongoc_rpc_validate_SOURCES = examples/mongoc-rpc-validate.c
 mongoc_rpc_validate_CFLAGS = -DMONGOC_COMPILATION $(EXAMPLE_CFLAGS)
 mongoc_rpc_validate_LDADD = libmongoc-priv.la
+if EXPLICIT_LIBS
+mongoc_rpc_validate_LDADD += $(BSON_LIBS)
+endif
 
 noinst_PROGRAMS += mongoc-tail
 mongoc_tail_SOURCES = examples/mongoc-tail.c